Python Programming/Collection

Python Programming

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Learning Guide

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This learning guide supports the Wikiversity course Python Programming, available at http://en.wikiversity.org/wiki/Python_Programming.

Overview

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Python Programming/Collection/Sidebar Python is a widely used high-level, general-purpose, interpreted, dynamic programming language. Python supports many programming paradigms, including object-oriented, functional, and procedural programming. Its design philosophy emphasizes code readability, simplicity, and its syntax allows programmers to express concepts in fewer lines of code than possible in other popular programming languages. Python is very extensible and the modularity of this language has made it popular for adding interfaces to existing applications.[1]

This course comprises 16 lessons on Python programming using Python 3. Each lesson includes a combination of Wikipedia and Internet-based readings, YouTube videos, and hands-on learning activities.

This entire Wikiversity course can be downloaded in book form by selecting Download Learning Guide in the sidebar.

Preparation

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This is a second-semester, college-level course. Learners should already be familiar with introductory computer concepts and have advanced or proficient-level computer skills.

Lessons

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  1. Introduction
  2. Variables
  3. Conditions
  4. Loops
  5. Functions
  6. Strings
  7. Lists
  8. Dictionaries
  9. Tuples and Sets
  10. Classes
  11. Modules
  12. RegEx
  13. Files
  14. Internet Data
  15. Databases
  16. GUI

See Also

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Participation

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This section is for the purpose of optionally documenting one's participation process in the course. Here might be notes by learners who might document their learning process and how they view the learning materials and/or how they interacted with them, contributed content or ideas regarding interacting with it. It may include participants experiences with material. Their motivation reasons for participating in the course as a whole and for parts of the course.

Bibliography

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References

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Lesson 1 - Introduction

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This lesson introduces the Python programming language and environment.

Objectives and Skills

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Objectives and skills for this lesson include:[1]

  • Introduction
    • Need and evolution of Python
    • Features of Python
    • The Prompt
    • Editor and source file
    • Translation and executable
  • Installation
    • Windows and Linux installation

Readings

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  1. Wikipedia: Python (programming language)
  2. Wikipedia: Integrated development environment
  3. Wikipedia: IDLE (Python)
  4. Python for Everyone: Why should you learn to write programs?

Multimedia

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  1. YouTube: Python for Informatics: Chapter 1 - Introduction by Chuck Severance
  2. YouTube: Python Programming by Derek Banas
  3. YouTube: Installing Python by thenewboston
  4. YouTube: Installing PyCharm by thenewboston
  5. YouTube: Python Basics Playlist (21 videos) by John Philip Jones
  6. YouTube: Learn Python - Full Course for Beginners by freeCodeCamp.org

Examples

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Comments

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A comment starts with a hash character (#) that is not part of a string literal, and ends at the end of the physical line.[2]

# This is a comment

Print

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The print function prints objects to standard output or to a text stream.[3]

# This script demonstrates a Python print statement.

print("Hello Wikiversity!")

Exit

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The exit function exits from Python.[4]

exit()

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Python

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  1. Learn the differences between Python 2 and Python 3.
    1. Review Python.org: Should I use Python 2 or Python 3 for my development activity?
  2. Download and install Python.
    1. Review TutorialsPoint: Python 3 Environment Setup.
    2. Download Python from Python.org: Downloads.
    3. Install Python.
    4. At a command prompt or terminal window, run the command python -V or python3 -V to confirm Python installation and display the version number.
  3. Use the Python interpreter.
    1. At a command prompt or terminal window, run the command python or python3 to start the Python interpreter.
    2. At the Python prompt, enter print("Hello <name>!"), replacing <name> with your name.
    3. At the Python prompt, enter exit() to exit Python.

Python IDEs

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  1. Use the Python IDLE integrated development environment.
    1. Review Wikipedia: IDLE (Python).
    2. Run IDLE on your system.
    3. At the Python prompt, enter print("Hello <name>!"), replacing <name> with your name.
    4. At the Python prompt, enter exit() to exit IDLE.
  2. Download and install another Python integrated development environment (IDE).
    1. Review TechAltair: 7 Best Python IDE for Pythonist.
    2. Select and download one of the recommended Python IDEs. Chose PyCharm Community Edition if you are unsure which one to select.
    3. Install the IDE.
    4. Run the IDE.
    5. In a new file, enter print("Hello <name>!"), replacing <name> with your name.
    6. Save the file as hello.py.
    7. Run the script and observe the results.
    8. Exit the IDE.
  3. Use a free cloud-based Python IDE.
    1. Review Wikipedia: Cloud9 IDE.
    2. Create a free account at Cloud9.
    3. Using Cloud9, create a new private workspace. Name the workspace whatever you like, or use python. Choose Python as the workspace template.
    4. Once the workspace starts, examine the Cloud9 IDE.
    5. Cloud9 has both Python 2 and Python 3 installed. At the bash prompt, run the command python to start the Python 2 interpreter.
    6. At the Python prompt, enter print("Hello <name>!"), replacing <name> with your name.
    7. At the Python prompt, enter exit() to exit Python.
    8. At the bash prompt, run the command python3 to start the Python 3 interpreter.
    9. At the Python prompt, enter print("Hello <name>!"), replacing <name> with your name.
    10. At the Python prompt, enter exit() to exit Python.
    11. Create a new file in the Cloud9 IDE.
    12. In the new file, enter print("Hello <name>!"), replacing <name> with your name.
    13. Save the file as hello.py.
    14. Run the script and observe the results.
    15. In the lower Run window, change the Runner to Python 3.
    16. Run the script and observe the results.
    17. To set Python 3 as the default runner, use Run / Run Configurations / Manage and set Python Version to Python 3.
    18. Exit Cloud9.

Games

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  1. Play CodeCombat Kithgard Dungeon levels 1 - 4.

Documentation

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  1. Read stackoverflow python questions: https://stackoverflow.com/questions/tagged/python?tab=Votes

Lesson Summary

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  • Python is a widely used high-level, general-purpose, interpreted, dynamic programming language.[5]
  • A high-level programming language is a programming language with strong abstraction from the details of the computer.[6]
  • An interpreted language is a programming language for which most of its implementations execute instructions directly, without previously compiling a program into machine-language instructions.[7]
  • There are three major versions of Python (1.x, 2.x, and 3.x). Python 2 and Python 3 are both considered current, stable languages. Python 3 is not backward-compatible with Python 2.[8]
  • Most Python implementations support a command-line interface / command-line interpreter in which users may enter statements sequentially and receive the results immediately. This sequence is known as a read–eval–print loop (REPL).[9]
  • The Python interpreter may also execute scripts, which are collections of Python source code saved as a file or files.[10]
  • A variety of Integrated Development Environments (IDEs) are available for Python. IDEs normally consist of a source code editor, build automation tools, and a debugger. Most modern IDEs also have intelligent code completion.[11]
  • IDLE (Integrated DeveLopment Environment) is an integrated development environment for Python, which has been bundled with the default implementation of the language since 1.5.2b1.[12]
  • A Python comment starts with a hash character (#) that is not part of a string literal, and ends at the end of the physical line.[13]
  • The print() function prints objects to standard output or to a text stream.[14]
  • The exit() function exits from Python.[15]
  • Python files use the filename extension .py.[16]

Key Terms

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bug
An error in a program.[17]
central processing unit
The heart of any computer. It is what runs the software that we write; also called “CPU” or “the processor”.[18]
compile
To translate a program written in a high-level language into a low-level language all at once, in preparation for later execution.[19]
high-level language
A programming language like Python that is designed to be easy for humans to read and write.[20]
interactive mode
A way of using the Python interpreter by typing commands and expressions at the prompt.[21]
interpret
To execute a program in a high-level language by translating it one line at a time.[22]
low-level language
A programming language that is designed to be easy for a computer to execute; also called “machine code” or “assembly language”.[23]
machine code
The lowest-level language for software, which is the language that is directly executed by the central processing unit (CPU).[24]
main memory
Stores programs and data. Main memory loses its information when the power is turned off.[25]
parse
To examine a program and analyze the syntactic structure.[26]
portability
A property of a program that can run on more than one kind of computer.[27]
print statement
An instruction that causes the Python interpreter to display a value on the screen.[28]
problem solving
The process of formulating a problem, finding a solution, and expressing the solution.[29]
program
A set of instructions that specifies a computation.[30]
prompt
When a program displays a message and pauses for the user to type some input to the program.[31]
secondary memory
Stores programs and data and retains its information even when the power is turned off. Generally slower than main memory. Examples of secondary memory include disk drives and flash memory in USB sticks.[32]
semantics
The meaning of a program.[33]
semantic error
An error in a program that makes it do something other than what the programmer intended.[34]
source code
A program in a high-level language.[35]

Review Questions

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Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. Python is _____.
    Python is a widely used high-level, general-purpose, interpreted, dynamic programming language.
  2. A high-level programming language is _____.
    A high-level programming language is a programming language with strong abstraction from the details of the computer.
  3. An interpreted language is _____.
    An interpreted language is a programming language for which most of its implementations execute instructions directly, without previously compiling a program into machine-language instructions.
  4. There are _____ major versions of Python. They are _____.
    There are three major versions of Python. They are 1.x, 2.x, and 3.x.
  5. Python 2 and Python 3 are both considered _____.
    Python 2 and Python 3 are both considered current, stable languages.
  6. Python 3 is _____-compatible with Python 2.
    Python 3 is not backward-compatible with Python 2.
  7. Most Python implementations support a command-line interface / command-line interpreter in which ____. This sequence is known as a ____.
    Most Python implementations support a command-line interface / command-line interpreter in which users may enter statements sequentially and receive the results immediately. This sequence is known as a read–eval–print loop (REPL).
  8. The Python interpreter may also execute _____.
    The Python interpreter may also execute scripts, which are collections of Python source code saved as a file or files.
  9. A variety of _____ (IDEs) are available for Python. IDEs normally consist of _____, _____ and _____. Most modern IDEs also have _____.
    A variety of Integrated Development Environments (IDEs) are available for Python. IDEs normally consist of a source code editor, build automation tools and a debugger. Most modern IDEs also have intelligent code completion.
  10. IDLE (Integrated DeveLopment Environment) is _____.
    IDLE (Integrated DeveLopment Environment) is an integrated development environment for Python, which has been bundled with the default implementation of the language since 1.5.2b1.
  11. A Python comment starts with _____, and ends _____.
    A Python comment starts with a hash character (#) that is not part of a string literal, and ends at the end of the physical line.
  12. The print() function _____.
    The print() function prints objects to standard output or to a text stream.
  13. The exit() function _____.
    The exit() function exits from Python.
  14. Python files use the filename extension _____.
    Python files use the filename extension .py.
  15. The difference between a high-level and low-level programming language is _____.
    A high-level programming language is easy for humans to understand, while a low-level programming language is harder to understand by a programmer but runs faster.

Assessments

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See Also

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References

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  1. Vskills: Certified Python Developer
  2. Python.org: Lexical analysis
  3. Python.org: Built-in Functions
  4. Python.org: System-specific parameters and functions
  5. Wikipedia:Python (programming language)
  6. Wikipedia: High-level programming language
  7. Wikipedia: Interpreted language
  8. Wikipedia:Python (programming language)
  9. Wikipedia:Python (programming language)
  10. Wikipedia: Scripting language
  11. Wikipedia: Integrated development environment
  12. Wikipedia: IDLE (Python)
  13. Python.org: Lexical analysis
  14. Python.org: Built-in Functions
  15. Python.org: System-specific parameters and functions
  16. Wikipedia:Python (programming language)
  17. PythonLearn: Why should you learn to write programs?
  18. PythonLearn: Why should you learn to write programs?
  19. PythonLearn: Why should you learn to write programs?
  20. PythonLearn: Why should you learn to write programs?
  21. PythonLearn: Why should you learn to write programs?
  22. PythonLearn: Why should you learn to write programs?
  23. PythonLearn: Why should you learn to write programs?
  24. PythonLearn: Why should you learn to write programs?
  25. PythonLearn: Why should you learn to write programs?
  26. PythonLearn: Why should you learn to write programs?
  27. PythonLearn: Why should you learn to write programs?
  28. PythonLearn: Why should you learn to write programs?
  29. PythonLearn: Why should you learn to write programs?
  30. PythonLearn: Why should you learn to write programs?
  31. PythonLearn: Why should you learn to write programs?
  32. PythonLearn: Why should you learn to write programs?
  33. PythonLearn: Why should you learn to write programs?
  34. PythonLearn: Why should you learn to write programs?
  35. PythonLearn: Why should you learn to write programs?

Lesson 2 - Variables

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This lesson introduces variables, expressions, and statements.


Objectives and Skills

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Objectives and skills for this lesson include:[1]

  • Language Basics
    • Language elements (constants, numbers and strings)
    • Strings types (single quotes, double quotes and triple quotes)
    • Escape Sequence, string concatenation and format method
    • Variables naming, types and objects
    • Indentation, logical and physical lines
  • Operators and Expressions
    • Operators and Expressions
    • Evaluation Order and Associativity
  • Input Output
    • User input

Readings

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  1. Wikipedia: Variable (computer science)
  2. Wikipedia: Data type
  3. Wikipedia: Expression (computer science)
  4. Wikipedia: Statement (computer science)
  5. Wikipedia: Order of operations
  6. Python for Everyone: Variables, expressions, and statements

Multimedia

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  1. YouTube: Python for Informatics: Chapter 2 - Expressions
  2. YouTube: Python Numbers
  3. YouTube: Python3 Input & Output
  4. Youtube: Python 3 Programming Tutorial: Variables
  5. Youtube: Setting up Pycharm and getting started

Examples

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Data Types

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Built-in Python data types include integer (int), floating point (float), string (str), and Boolean (bool) data types.[2]

value = 1 + 1
print(value)          # Displays 2
print(type(value))    # Displays <type 'int'>

value = 0.1 + 0.1
print(value)          # Displays 0.2 
print(type(value))    # Displays <type 'float'>

value = '1' + '1'
print(value)          # Displays 11
print(type(value))    # Displays <type 'str'>

value = True
print(value)          # Displays True
print(type(value))    # Displays <type 'bool'>

Type Conversion

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An object’s type is accessed by the built-in function type().[3]

value = 1.9
print(value)          # Displays 1.9
print(type(value))    # Displays <type 'float'>
value = int(value)
print(value)          # Displays 1
print(type(value))    # Displays <type 'int'>

value = 1
print(value)          # Displays 1
print(type(value))    # Displays <type 'int'>
value = float(value)  
print(value)          # Displays 1.0
print(type(value))    # Displays <type 'float'>

value = 1
print(value)          # Displays 1
print(type(value))    # Displays <type 'int'>
value = str(value)  
print(value)          # Displays 1
print(type(value))    # Displays <type 'str'>

Quotes

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String literals are written in a variety of ways, including single quotes, double quotes, and triple quotes. Triple quoted strings may span multiple lines.[4] The backslash (\) character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character.[5]

value = 'single quotes'
print(value)

value = "double quotes"
print(value)

value = \
'''triple quotes
    span multiple lines'''
print(value)

value = '"nested quotes"'
print(value)

value = "'nested quotes'"
print(value)

value = "\"escape character quotes\nand multiple lines\""
print(value)

Numeric Operations

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All numeric types (except complex) support the following operations, sorted by ascending priority.[6]

a = 3
b = 2

print(a + b)     # 5
print(a - b)     # 1
print(a * b)     # 6
print(a / b)     # 1.5
print(a // b)    # 1
print(a % b)     # 1
print(-a)        # -3
print(a ** b)    # 9

Assignment Operations

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An assignment statement evaluates the expression and assigns the result to the target. Augmented assignment is the combination, in a single statement, of an operation and an assignment statement.[7]

a = 3
b = 2

a += b    # a = 5
a -= b    # a = 3
a *= b    # a = 6
a /= b    # a = 1.5
a //= b   # a = 1
a %= b    # a = 1.0
a **= b   # a = 9

Input Function

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Python 2: If the prompt argument is present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), which is then parsed and evaluated as a Python expression.[8]

Python 3: If the prompt argument is present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that.[9]

input([prompt])

#Python 2
value = input("Enter a numeric value: ")
print("You entered " + str(value))

value = input('Enter a string value in "quotes": ')
print("You entered " + value)

#Python 3
value = input("Enter a value: ")
print("You entered a string value " + value)

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Practice

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  1. Experiment with different numeric operations to ensure you understand how they work. Then review either MathsIsFun: Order of Operations or Teachoo: What is BODMAS?. Create a Python program that demonstrates the order of operations.
  2. Create a Python program to prompt the user for hours and rate per hour to compute gross pay (hours * rate).[10]
  3. Review MathsIsFun: Conversion of Temperature. Create a Python program that asks the user for a Fahrenheit temperature and then calculate and display the corresponding Celsius temperature or ask the user for a Celsius temperature and then calculate and display the corresponding Fahrenheit temperature.
  4. Create a Python program that asks the user how old they are in years, and then calculate and display their approximate age in months, days, hours, and seconds.
  5. Review MathsIsFun: Area of Plane Shapes. Create a Python program that asks the user for the dimensions of different shapes and then calculate and display the area of the shapes.

Games

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  1. Play CodeCombat.

Lesson Summary

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  • Built-in Python data types include integer (int()), floating point (float()), string (str()), and Boolean (bool()).[11]
  • An object’s type is accessed by the built-in function type().[12]
  • String literals are written in a variety of ways, including single quotes, double quotes, and triple quotes. Triple quoted strings may span multiple lines.[13]
  • The backslash (\) character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character.[14]
  • Numeric operators include +, -, *, /, //, %, and **.[15]
  • Assignment operators include +=, -=, *=, /=, //=, %=, and **=.[16]
  • The Python 2 input() function reads a line from input, converts it to a string (stripping a trailing newline), which is then parsed and evaluated as a Python expression.[17]
  • The Python 3 input() function reads a line from input, converts it to a string (stripping a trailing newline), and returns that.[18]

Key Terms

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assignment
A statement that assigns a value to a variable.[19]
concatenate
To join two operands end to end.[20]
comment
Information in a program that is meant for other programmers (or anyone reading the source code) and has no effect on the execution of the program.[21]
escape character
A character which invokes an alternative interpretation on subsequent characters in a character sequence.[22]
evaluate
To simplify an expression by performing the operations in order to yield a single value.[23]
expression
A combination of variables, operators, and values that represents a single result value.[24]
floating point
A type that represents numbers with fractional parts.[25]
floor division
The operation that divides two numbers and chops off the fractional part.[26]
integer
A type that represents whole numbers.[27]
keyword
A reserved word that is used by the compiler to parse a program; you cannot use keywords like if, def, and while as variable names.[28]
mnemonic
A memory aid. We often give variables mnemonic names to help us remember what is stored in the variable.[29]
modulus operator
An operator, denoted with a percent sign (%), that works on integers and yields the remainder when one number is divided by another.[30]
operand
One of the values on which an operator operates.[31]
operator
A special symbol that represents a simple computation like addition, multiplication, or string concatenation.[32]
rules of precedence
The set of rules governing the order in which expressions involving multiple operators and operands are evaluated.[33]
statement
A section of code that represents a command or action. So far, the statements we have seen are assignments and print statements.[34]
string
A type that represents sequences of characters.[35]
type
A category of values. The types we have seen so far are integers (type int), floating-point numbers (type float), and strings (type str).[36]
value
One of the basic units of data, like a number or string, that a program manipulates.[37]
variable
A name that refers to a value.[38]

Review Questions

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Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. Built-in Python data types include _____, _____, _____, and _____.
    Built-in Python data types include integer (int()), floating point (float()), string (str()), and Boolean (bool()).
  2. An object’s type is accessed by the built-in function _____.
    An object’s type is accessed by the built-in function type().
  3. String literals are written in a variety of ways, including _____, _____, and _____. _____ may span multiple lines.
    String literals are written in a variety of ways, including single quotes, double quotes, and triple quotes. Triple quoted strings may span multiple lines.
  4. The _____ character is used to escape characters that otherwise have a special meaning, such as _____.
    The backslash (\) character is used to escape characters that otherwise have a special meaning, such as newline, backslash itself, or the quote character.
  5. Numeric operators include _____.
    Numeric operators include +, -, *, /, //, %, and **.
  6. Assignment operators include _____.
    {{{2}}}
  7. The Python 2 input() function _____.
    The Python 2 input() function reads a line from input, converts it to a string (stripping a trailing newline), which is then parsed and evaluated as a Python expression.
  8. The Python 3 input() function _____.
    The Python 3 input() function reads a line from input, converts it to a string (stripping a trailing newline), and returns that.

Assessments

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See Also

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References

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  1. Vskills: Certified Python Developer
  2. Python.org Built-in Types
  3. Python.org: Built-in Types
  4. Python.org: Built-in Types
  5. Python.org: Lexical analysis
  6. Python.org: Built-in Types
  7. Python.org: Simple Statements
  8. Python.org: Built-in Functions
  9. Python.org: Built-in Functions
  10. PythonLearn: Variables, expressions, and statements
  11. Python.org Built-in Types
  12. Python.org: Built-in Types
  13. Python.org: Built-in Types
  14. Python.org: Lexical analysis
  15. Python.org: Built-in Types
  16. Python.org: Simple Statements
  17. Python.org: Built-in Functions
  18. Python.org: Built-in Functions
  19. PythonLearn: Variables, expressions, and statements
  20. PythonLearn: Variables, expressions, and statements
  21. PythonLearn: Variables, expressions, and statements
  22. Wikipedia: Escape character
  23. PythonLearn: Variables, expressions, and statements
  24. PythonLearn: Variables, expressions, and statements
  25. PythonLearn: Variables, expressions, and statements
  26. PythonLearn: Variables, expressions, and statements
  27. PythonLearn: Variables, expressions, and statements
  28. PythonLearn: Variables, expressions, and statements
  29. PythonLearn: Variables, expressions, and statements
  30. PythonLearn: Variables, expressions, and statements
  31. PythonLearn: Variables, expressions, and statements
  32. PythonLearn: Variables, expressions, and statements
  33. PythonLearn: Variables, expressions, and statements
  34. PythonLearn: Variables, expressions, and statements
  35. PythonLearn: Variables, expressions, and statements
  36. PythonLearn: Variables, expressions, and statements
  37. PythonLearn: Variables, expressions, and statements
  38. PythonLearn: Variables, expressions, and statements

Lesson 3 - Conditions

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This lesson introduces conditions and exception handling.


Objectives and Skills

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Objectives and skills for this lesson include:[1]

  • Control Flow
    • The if statement
  • Exceptions
    • Errors and exceptions
    • Handling and raising Exceptions
    • Try, Finally and the with statement

Readings

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  1. Wikipedia: Conditional (computer programming)
  2. Wikipedia: Exception handling
  3. Python for Everyone: Conditional execution

Multimedia

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  1. YouTube: Python Tutorial for Beginners 6: Conditionals and Booleans - If, Else, and Elif Statements
  2. YouTube: Python for Informatics - Chapter 3 - Conditional Execution
  3. YouTube: If, Then, Else in Python
  4. YouTube: Python if elif else
  5. YouTube: How to Use If Else Statements in Python

Examples

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Comparison Operations

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There are eight comparison operations in Python.[2]

a = 3
b = 2

print(a == b)       # False
print(a != b)       # True
print(a < b)        # False
print(a > b)        # True
print(a <= b)       # False
print(a >= b)       # True
print(a is b)       # False
print(a is not b)   # True

Boolean Operations

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The Boolean operations include and, or, and not.[3]

a = 3
b = 2

print(a < b and b < a)    # False
print(a < b or b < a)     # True
print(a < b)              # False
print(not(a < b))         # True

Bitwise Operations

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Bitwise operations only make sense for integers. Negative numbers are treated as their 2’s complement value.[4]

a = 5
b = 3

print(bin(a))        # 0101
print(bin(b))        # 0011
print(bin(a & b))    # 0001
print(bin(a | b))    # 0111
print(bin(a ^ b))    # 0110
print(bin(~a))       # -0110
print(bin(a << b))   # 101000
print(bin(a >> b))   # 000000

If Statement

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An if statement may contain zero or more elif parts, and the else part is optional. The keyword ‘elif‘ is short for ‘else if’, and is useful to avoid excessive indentation. An if ... elif ... elif ... sequence is a substitute for the switch or case statements found in other languages.[5]

input = input("Is it morning (m), afternoon (a), or evening (e)? ")
if input == "m":
    print("Good morning!")
elif input == "a":
    print("Good afternoon!")
elif input == "e":
    print("Good evening!")
else:
    print("Hello!")

Try Statement

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The try statement executes the try clause. If no exception occurs, the except clause is skipped. If an exception occurs during execution of the try clause, the except clause is executed, and then execution continues after the try statement.[6]

try:
    value = input("Enter a numeric value: ")
    result = 1 / float(value)
    print("1 / " + value + " = " + str(result))
except:
    print("An error occurred dividing 1 by " + value + "!")

Except Statement

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A try statement may have more than one except clause, to specify handlers for different exceptions.[7]

try:
    value = input("Enter a numeric value: ")
    result = 1 / float(value)
    print("1 / " + value + " = " + str(result))
except ValueError as error:
    print(value + " is not a numeric value!")
except ZeroDivisionError as error:
    print("Cannot divide by 0!")
except:
    print("An unexpected error occurred dividing 1 by " + value + "!")

Activities

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Tutorials

edit
  1. Complete one or more of the following tutorials:

Practice

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  1. Review Python.org: Truth Value Testing. Create a Python program that uses different values and conditions to confirm that in Python zero is false and anything non-zero is true. Also show that the explicit value of False is 0 and the explicit value of True is 1.
  2. Create a Python program that asks the user how old they are in years. Then ask the user if they would like to know how old they are in months, days, hours, or seconds. Use an if/elif/else statement to display their approximate age in the selected timeframe.
  3. Create a Python program to prompt the user for hours and rate per hour to compute gross pay (hours * rate). Include a calculation to give 1.5 times the hourly rate for any overtime (hours worked above 40 hours).[8]
  4. Review MathsIsFun: Conversion of Temperature. Create a Python program that asks the user if they would like to convert Fahrenheit to Celsius or Celsius to Fahrenheit. Use an if/elif/else statement to determine their selection and then gather the appropriate input and calculate and display the converted temperature.
  5. Review MathsIsFun: Area of Plane Shapes. Create a Python program that asks the user what shape they would like to calculate the area for. Use an if/elif/else statement to determine their selection and then gather the appropriate input and calculate and display the area of the shape.
  6. Extend one or more of the programs above by adding a try/except block to handle any runtime errors caused by the user entering invalid values for the input.

Games

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  1. Play CodeCombat.

Lesson Summary

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Condition Concepts

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  • Conditional statements are features of a programming language which perform different computations or actions depending on whether a programmer-specified boolean condition evaluates to true or false.[9]
  • The pseudocode structure of a conditional statement is:[10]
IF (boolean condition) THEN
    (consequent)
ELSE
    (alternative)
END IF
  • If the condition is true, the statements following the THEN are executed. Otherwise, the execution continues in the following branch – either in the ELSE block (which is usually optional), or if there is no ELSE branch, then after the END IF.[11]
  • By using ELSE IF/ELSEIF, it is possible to combine several conditions. Only the statements following the first condition that is found to be true will be executed. All other statements will be skipped.[12]
  • Exception handling is the process of responding to the occurrence, during computation, of anomalous or exceptional events requiring special processing, often changing the normal flow of program execution.[13]
  • In programming language mechanisms for exception handling, the term exception is typically used in a specific sense to denote a data structure storing information about an exceptional condition.[14]
  • One mechanism to transfer control, or raise an exception, is known as a throw.[15]
  • The scope for exception handlers starts with a "try" clause.[16]
  • An exception is said to be thrown and execution is transferred to a "catch" or "except" statement.[17][18]

Python Conditions

edit
  • The Python syntax for a conditional statement is:[19]
if <condition>:
    <statements>
elif <condition>:
    <statements>
else:
    <statements>
  • An if statement may contain zero or more elif parts, and the else part is optional.[20]
  • An if ... elif ... elif ... sequence is a substitute for the switch or case statements found in other languages.[21]
  • There are eight comparison operations in Python, including ==, !=, <, >, <=, >=, is, is not.[22]
  • There are three Boolean operations in Python, including and, or, not.[23]
  • There are six bitwise operations in Python, including &, |, ^, ~, <<, >>.[24]
  • Bitwise operations only make sense for integers. Negative numbers are treated as their 2’s complement value.[25]
  • The Python syntax for a try statement is:[26]
try:
    <statements>
except <exception>:
    <statements>
except:
    <statements>
  • The try statement executes the try clause. If f no exception occurs, the except clause is skipped. If an exception occurs during execution of the try clause, the except clause is executed, and then execution continues after the try statement.[27]
  • A try statement may have more than one except clause, to specify handlers for different exceptions.[28]

Key Terms

edit
body
The sequence of statements within a compound statement.[29]
boolean expression
An expression whose value is either True or False.[30]
branch
One of the alternative sequences of statements in a conditional statement.[31]
chained conditional
A conditional statement with a series of alternative branches.[32]
comparison operator
One of the operators that compares its operands ==, !=, >, <, >=, and <=.[33]
conditional statement
A statement that controls the flow of execution depending on some condition.[34]
condition
The boolean expression in a conditional statement that determines which branch is executed.[35]
compound statement
A statement that consists of a header and a body. The header ends with a colon (). The body is indented relative to the header.[36]
guardian pattern
Where we construct a logical expression with additional comparisons to take advantage of the short-circuit behavior.[37]
logical operator
One of the operators that combines boolean expressions and, or, and not.[38]
nested conditional
A conditional statement that appears in one of the branches of another conditional statement.[39]
traceback
A list of the functions that are executing, printed when an exception occurs.[40]
short circuit
When Python is part-way through evaluating a logical expression and stops the evaluation because Python knows the final value for the expression without needing to evaluate the rest of the expression.[41]

Review Questions

edit
Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. Conditional statements are features of a programming language which _____.
    Conditional statements are features of a programming language which perform different computations or actions depending on whether a programmer-specified boolean condition evaluates to true or false.
  2. The pseudocode structure of a conditional statement is:
    The pseudocode structure of a conditional statement is:

    IF (boolean condition) THEN
        (consequent)
    ELSE
        (alternative)
    END IF

  3. If a condition is true, _____. Otherwise, the execution continues _____.
    If the condition is true, the statements following the THEN are executed. Otherwise, the execution continues in the following branch – either in the ELSE block (which is usually optional), or if there is no ELSE branch, then after the END IF.
  4. By using ELSE IF/ELSEIF, it is possible to _____.
    By using ELSE IF/ELSEIF, it is possible to combine several conditions. Only the statements following the first condition that is found to be true will be executed. All other statements will be skipped.
  5. Exception handling is the process of _____.
    Exception handling is the process of responding to the occurrence, during computation, of anomalous or exceptional events requiring special processing, often changing the normal flow of program execution.
  6. In programming language mechanisms for exception handling, the term exception is typically used in a specific sense to denote _____.
    In programming language mechanisms for exception handling, the term exception is typically used in a specific sense to denote a data structure storing information about an exceptional condition.
  7. One mechanism to transfer control, or raise an exception, is known as _____.
    One mechanism to transfer control, or raise an exception, is known as a throw.
  8. The scope for exception handlers starts with _____.
    The scope for exception handlers starts with a "try" clause.
  9. An exception is said to be thrown and execution is transferred to _____.
    An exception is said to be thrown and execution is transferred to a "catch" or "except" statement.
  10. The Python syntax for a conditional statement is:
    The Python syntax for a conditional statement is:

    if <condition>:
        <statements>
    elif <condition>:
        <statements>
    else:
        <statements>

  11. An if statement may contain zero or more _____ parts, and the _____ part is optional.
    An if statement may contain zero or more elif parts, and the else part is optional.
  12. An if ... elif ... elif ... sequence is a substitute for _____ found in other languages.
    An if ... elif ... elif ... sequence is a substitute for the switch or case statements found in other languages.
  13. There are eight comparison operations in Python, including _____.
    There are eight comparison operations in Python, including ==, !=, <, >, <=, >=, is, is not.
  14. There are three Boolean operations in Python, including _____.
    There are three Boolean operations in Python, including and, or, not.
  15. There are six bitwise operations in Python, including _____.
    There are six bitwise operations in Python, including &, |, ^, ~, <<, >>.
  16. Bitwise operations only make sense for _____. Negative numbers are treated as _____.
    Bitwise operations only make sense for integers. Negative numbers are treated as their 2’s complement value.
  17. The Python syntax for a try statement is:
    The Python syntax for a try statement is:

    try:
        <statements>
    except <exception>:
        <statements>
    except:
        <statements>

  18. The try statement executes _____. If f no exception occurs, _____. If an exception occurs during execution _____.
    The try statement executes the try clause. If f no exception occurs, the except clause is skipped. If an exception occurs during execution of the try clause, the except clause is executed, and then execution continues after the try statement.
  19. A try statement may have more than one _____.
    A try statement may have more than one except clause, to specify handlers for different exceptions.

Assessments

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See Also

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References

edit
  1. Vskills: Certified Python Developer
  2. Python.org: Built-in Types
  3. Python.org: Built-in Types
  4. Python.org: Built-in Types
  5. Python.org: More Control Flow Tools
  6. Python.org: Errors and Exceptions
  7. Python.org: Errors and Exceptions
  8. PythonLearn: Conditional Execution
  9. Wikipedia: Conditional (computer programming)
  10. Wikipedia: Conditional (computer programming)
  11. Wikipedia: Conditional (computer programming)
  12. Wikipedia: Conditional (computer programming)
  13. Wikipedia: Exception handling
  14. Wikipedia: Exception handling
  15. Wikipedia: Exception handling
  16. Wikipedia: Exception handling
  17. Wikipedia: Exception handling
  18. Python.org Handling Exceptions
  19. Python.org: More Control Flow Tools
  20. Python.org: More Control Flow Tools
  21. Python.org: More Control Flow Tools
  22. Python.org: Built-in Types
  23. Python.org: Built-in Types
  24. Python.org: Built-in Types
  25. Python.org: Built-in Types
  26. Python.org Handling Exceptions
  27. Python.org: Errors and Exceptions
  28. Python.org: Errors and Exceptions
  29. PythonLearn: Conditional execution
  30. PythonLearn: Conditional execution
  31. PythonLearn: Conditional execution
  32. PythonLearn: Conditional execution
  33. PythonLearn: Conditional execution
  34. PythonLearn: Conditional execution
  35. PythonLearn: Conditional execution
  36. PythonLearn: Conditional execution
  37. PythonLearn: Conditional execution
  38. PythonLearn: Conditional execution
  39. PythonLearn: Conditional execution
  40. PythonLearn: Conditional execution
  41. PythonLearn: Conditional execution

Lesson 4 - Loops

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This lesson introduces loops.


Objectives and Skills

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Objectives and skills for this lesson include:[1]

  • Control Flow
    • The while statement
    • The for loop
    • The break and continue statement

Readings

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  1. Wikipedia: Control flow
  2. Python for Everyone: Iteration
  3. Python Basics: Iteration, Iterables, Iterators, and Looping

Multimedia

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  1. YouTube: Python for Informatics: Chapter 5 - Iterations
  2. YouTube: Python For
  3. YouTube: Python Range and While
  4. YouTube: Python Break
  5. YouTube: Python Continue
  6. YouTube: Python Nested Loops
  7. YouTube: Python Iteration Playlist
  8. Python Tutorial for Beginners 7: Loops and Iterations - For/While Loops

Examples

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While Statement

edit

The while loop executes as long as the condition remains true. In Python, any non-zero integer value is true and zero is false. The condition may also be a string or list value, in which case anything with a non-zero length is true and empty sequences are false. The body of the loop must be indented, and each line within a block must be indented by the same amount.[2]

i = 0
while i < 3:
    print(i)    # 0, 1, 2
    i += 1

For Statement

edit

The for statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence.[3]

for i in range(3):
    print(i)    # 0, 1, 2

Range Function

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The range function returns an immutable sequence of numbers and is commonly used for looping a specific number of times in for loops. The syntax for the range function is range([start,] stop[, step]), with start defaulting to 0 and step defaulting to 1.[4]

for i in range(0, 10, 2):
    print(i)    # 0, 2, 4, 6, 8

Break Statement

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The break statement breaks out of the smallest enclosing for or while loop.[5]

i = 0
while True:
    print(i)    # 0, 1, 2
    i += 1
    if i >= 3:
        break

Continue Statement

edit

The continue statement continues with the next iteration of the loop.[6]

for i in range(10):
    if i % 2:
        continue
    print(i)    # 0, 2, 4, 6, 8

Nested Loops

edit

Loops may be nested with one loop inside another.[7]

for i in range(1, 4):
    for j in range(1, 4):
        print(str(i) + str(j), end = '\t')
    print()
print()

i = 1
while i < 4:
    j = 1
    while j < 4:
        print(str(i) + str(j), end = '\t')
        j += 1
    print()
    i += 1

Output:

11      12      13
21      22      23
31      32      33

11      12      13
21      22      23
31      32      33

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Practice

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  1. Review MathsIsFun: Definition of Average. Create a Python program that asks the user to enter grade scores. Start by asking the user how many scores they would like to enter. Then use a loop to request each score and add it to a total. Finally, calculate and display the average for the entered scores. Include try and except to handle input errors. Revise the program to compare the code required when the loop control structure is based on both while and for statements.
  2. Create a Python program that uses a loop to generate a list of multiplication expressions for a given value. Ask the user to enter the value and the number of expressions to be displayed. Include try and except to handle input errors. For example, a list of three expressions for the value 1 would be:
        1 * 1 = 1
        1 * 2 = 2
        1 * 3 = 3
  3. Review MathsIsFun: 10x Printable Multiplication Table. Create a Python program that uses nested loops to generate a multiplication table. Rather than simply creating a 10 by 10 table, ask the user to enter the starting and ending values. Include row and column labels, and include try and except to handle input errors. For example, the output might look like:
            1   2   3
        1   1   2   3
        2   2   4   6
        3   3   6   9
  4. Review MathsIsFun: 10x Printable Multiplication Table. Create a Python program that uses nested loops to generate a multiplication table. Rather than simply creating a 10 by 10 table, ask the user to enter the starting and ending values. Use a condition with (<variable> % 2) to test for odd or even values. Generate the multiplication table only for even values. For odd values, use the continue statement to continue the next iteration of the loop. Include row and column labels, and include try and except to handle input errors. For example, the output might look like:
            2   4   6
        2   4   8  12
        4   8  16  24
        6  12  24  36
  5. Review MathsIsFun: 10x Printable Multiplication Table. Create a Python program that uses nested loops to generate a multiplication table. Rather than simply creating a 10 by 10 table, ask the user to enter the starting and ending values. Use a break statement to terminate the loop if the number of rows or columns exceeds 10. Include row and column labels, and include try and except to handle input errors.

Games

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  1. Play CodeCombat.

Lesson Summary

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Control Flow Concepts

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  • Control flow refers to the order in which the individual statements, instructions or function calls of an imperative or a declarative program are executed or evaluated.[8]
  • Control flow statement types include unconditional branch, conditional branch, conditional loop, subroutines, and unconditional halt.[9]
  • Python does not support an unconditional branch. All control flow must be structured using conditions, loops. functions, or exit (unconditional halt).[10]
  • Python uses colons and indentation to designate code blocks and control structures.[11]
  • A loop is a sequence of statements which is specified once but which may be carried out several times in succession.[12]

Python Loops

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  • Python loop structures include while and for.[13]
  • While is a condition-controlled loop, repeating until some condition changes.[14]
  • Python for loops are collection-controlled loops repeating for all elements of a sequence, which is more like foreach in other programming languages.[15]
  • Loops may be continued prematurely using the continue statement.[16]
  • Loops may be terminated prematurely using the break statement.[17]
  • Programs may be terminated prematurely using the exit() function.[18]
  • The while loop executes as long as the condition remains true. In Python, any non-zero integer value is true and zero is false. The condition may also be a string or list value, in which case anything with a non-zero length is true and empty sequences are false.[19]
  • The body of the loop must be indented, and each line within a block must be indented by the same amount.[20]
  • The syntax for the while loop is:[21]
while condition:
    statements
  • The for statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence.[22]
  • The syntax for the for loop is:[23]
for variable in sequence:
    statements
  • The range function returns an immutable sequence of numbers and is commonly used for looping a specific number of times in for loops.[24]
  • The syntax for the range function is range([start,] stop[, step]), with start defaulting to 0 and step defaulting to 1.[25]
  • The break statement breaks out of the smallest enclosing for or while loop.[26]
  • The continue statement continues with the next iteration of the loop.[27]

Key Terms

edit
accumulator
A variable used in a loop to add up or accumulate a result.[28]
counter
A variable used in a loop to count the number of times something happened. We initialize a counter to zero and then increment the counter each time we want to “count” something.[29]
decrement
An update that decreases the value of a variable.[30]
initialize
An assignment that gives an initial value to a variable that will be updated.[31]
increment
An update that increases the value of a variable (often by one).[32]
infinite loop
A loop in which the terminating condition is never satisfied or for which there is no terminating condition.[33]
iteration
Repeated execution of a set of statements using either a function that calls itself or a loop.[34]

Review Questions

edit
Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. Control flow refers to _____.
    Control flow refers to the order in which the individual statements, instructions or function calls of an imperative or a declarative program are executed or evaluated.
  2. Control flow statement types include _____, _____, _____, _____, and _____.
    Control flow statement types include unconditional branch, conditional branch, conditional loop, subroutines, and unconditional halt.
  3. Python does not support _____. All control flow must be structured using _____.
    Python does not support an unconditional branch. All control flow must be structured using conditions, loops. functions, or exit (unconditional halt).
  4. Python uses _____ to designate code blocks and control structures.
    Python uses colons and indentation to designate code blocks and control structures.
  5. A loop is _____.
    A loop is a sequence of statements which is specified once but which may be carried out several times in succession.
  6. Python loop structures include _____ and _____.
    Python loop structures include while and for.
  7. While is _____.
    While is a condition-controlled loop, repeating until some condition changes.
  8. Python for loops are _____, which is more like _____ in other programming languages.
    Python for loops are collection-controlled loops repeating for all elements of a sequence, which is more like foreach in other programming languages.
  9. Loops may be continued prematurely using the _____ statement.
    Loops may be continued prematurely using the continue statement.
  10. Loops may be terminated prematurely using the _____ statement.
    Loops may be terminated prematurely using the break statement.
  11. Programs may be terminated prematurely using the _____ function.
    Programs may be terminated prematurely using the exit() function.
  12. The while loop executes as long as _____. In Python, _____ is true and _____ is false. The condition may also be _____, in which case _____ is true and _____ are false.
    The while loop executes as long as the condition remains true. In Python, any non-zero integer value is true and zero is false. The condition may also be a string or list value, in which case anything with a non-zero length is true and empty sequences are false.[18]
  13. The body of the loop must be _____.
    The body of the loop must be indented, and each line within a block must be indented by the same amount.
  14. The syntax for the while loop is:
    The syntax for the while loop is:

    while condition:
        statements

  15. The for statement _____.
    The for statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence.
  16. The syntax for the for loop is:
    The syntax for the for loop is:

    for variable in sequence:
        statements

  17. The range function _____.
    The range function returns an immutable sequence of numbers and is commonly used for looping a specific number of times in for loops.
  18. The syntax for the range function is _____.
    The syntax for the range function is range([start,] stop[, step]), with start defaulting to 0 and step defaulting to 1.
  19. The break statement _____.
    The break statement breaks out of the smallest enclosing for or while loop.
  20. The continue statement _____.
    The continue statement continues with the next iteration of the loop.

Assessments

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See Also

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References

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Lesson 5 - Functions

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This lesson introduces functions.


Objectives and Skills

edit

Objectives and skills for this lesson include:[1]

  • Functions
    • Function parameters and local variables
    • Using global and nonlocal statement
    • Default Argument values and keyword arguments
    • VarArgs and keyword-only parameters
    • The return statement
    • DocStrings and annotations

Readings

edit
  1. Wikipedia: Function (computer science)
  2. Wikipedia: Parameter (computer programming)
  3. Wikipedia: Recursion (computer science)
  4. Python for Everyone: Functions

Multimedia

edit
  1. YouTube: Python for Informatics - Chapter 4 - Functions
  2. YouTube: Python Functions
  3. YouTube: Python Return Values
  4. YouTube: Variable Scope
  5. YouTube: Default Values for Arguments
  6. YouTube: Keyword Arguments
  7. YouTube: Flexible Number of Arguments
  8. YouTube: Python 3 Programming Tutorial - Function Parameters
  9. YouTube: Python 3.5 Tutorial - Explaining The DocString
  10. YouTube: How To Use Functions In Python (Python Tutorial #3)

Examples

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Built-In Functions

edit

The Python interpreter has a number of functions and types built into it that are always available.[2]

value = 65
print(bin(value))    # 0b1000001
print(oct(value))    # 0o101
print(hex(value))    # 0x41
print(chr(value))    # A

value = 1234.567
print(round(value, 1))          # 1234.6
print(format(value, ",.2f"))    # 1,234.57

User-Defined Functions

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A function definition defines a user-defined function object.[3] If specified, return leaves the current function call with the expression list (or None) as return value.[4]

def function_1():
    print("In function_1")
    
def function_2(parameter):
    print("In function_2 with parameter value:", parameter)
    
def function_3(parameter):
    print("In function_3 with parameter value:", parameter)
    return(parameter * 2)
    
function_1()
function_2("test")
result = function_3("test")
print("function_3 returned", result)

Output:

In function_1
In function_2 with parameter value: test
In function_3 with parameter value: test
function_3 returned testtest

Local Variables

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If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.[5]

def local_variable():
    value = 1
    print("Local value:", value)

def local_parameter(value):
    print("Parameter value:", value)
    value = 2
    print("Parameter value:", value)

local_variable()
try:
    print("value:", value)
except:
    print("Cannot access local variable.")
    
value = 1
local_parameter(value)
print("Global value:", value)

Output:

Local value: 1
Cannot access local variable.
Parameter value: 1
Parameter value: 2
Global value: 1

Global Variables

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Variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.[6]

def global_access():
    print("Global value:", value)

def global_modification():
    global value
    value = 2
    print("Global value:", value)

value = 1
global_access()
global_modification()
print("Global value:", value)

Output:

Global value: 1
Global value: 2
Global value: 2

Nonlocal Variables

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The nonlocal statement causes the listed identifiers to refer to previously bound variables in the nearest enclosing scope excluding globals.[7][8]

global_value = 1

def function():
    function_value = 2
    
    def nested_function():
        global global_value
        nonlocal function_value
        
        global_value = 3
        function_value = 4
    
    nested_function()
    print("Global value:", global_value)
    print("Function value:", function_value)

function()

Output:

Global value: 3
Function value: 4

Parameters

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A parameter is a named entity in a function definition that specifies an argument that the function can accept.[9] Functions may specify a default value for one or more arguments.[10] Functions may also be called using keyword arguments rather than positional arguments.[11]

def parameters(x = 1, y = 2):
    print("x =", x, "y =", y)
    
parameters()
parameters(3, 4)
parameters(5, 6)
parameters(y = 7, x = 8)

Output:

x = 1 y = 2
x = 3 y = 4
x = 5 y = 6
x = 8 y = 7

Parameter Validation

edit

According to the Zen of Python, errors should never pass silently.[12] Parameter validation is automated processing to validate the spelling or accuracy of parameters passed to a function or module.[13]

def function(integer):
    if type(integer) is not int:
        raise TypeError("Parameter integer must be an integer.")

    if integer < 0:
        raise ValueError("Parameter integer must be greater than or equal to zero.")
    ...

Assertions

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An assertion is a statement that a true–false expression is expected to always be true at that point in the code.[14] The assert statement verifies that an expression is true or it raises an AssertionError. Unlike conditions, which are always executed, assert statements may be disabled when optimization is requested (using command line option -O).[15]

def function(integer):
    assert type(integer) is int
    assert integer >= 0
    ...

Variable Arguments

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Functions may accept a variable number of arguments using the * operator.[16] The variable arguments are provided as a Python list. See Python Programming/Lists for more information.

def sum(*args):
    total = 0
    for arg in args:
        total += arg
    return total
    
print(sum(1, 2, 3, 4))

Output:

10

Variable Named Arguments

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Variable named arguments may be accessed using the ** operator.[17][18][19] The variable named arguments are provided as a Python dictionary. See Python Programming/Dictionaries for more information.

def keywords(**kwargs):
    for item in kwargs.items():
        print(item)

keywords(x = 1, y = 2)

Output:

('y', 2)
('x', 1)

Docstring

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A docstring is a string literal that occurs as the first statement in a module, function, class, or method definition. Such a docstring becomes the __doc__ special attribute of that object. For consistency, always use """triple double quotes""" around docstrings.[20][21]

def function(parameter):
    """This docstring documents the function, describes the parameter requirements, etc."""
    pass

print(function.__doc__)

Output:

This docstring documents the function, describes the parameter requirements, etc.

Recursion Example

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The factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n.[22]

def factorial(value):
    """This function returns the factorial of the value provided using recursion."""

    if type(value) is not int:
        raise TypeError("Parameter value must be an integer.")

    if value < 0:
        result = 0
    elif value == 0:
        result = 1
    else:
        result = value * factorial(value - 1)
    return result

print("5! =", factorial(5))

Output:

5! = 120

Activities

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Tutorials

edit
  1. Complete one or more of the following tutorials:

Practice

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  1. Review Python.org: Built-in Functions. Create a Python program that asks the user for a numeric input value. Demonstrate the use of a variety of built-in functions by displaying the input value's initial type (type). Then convert and display the input value as an integer (int), a float (float), and a Boolean (bool). Use the converted integer to display the value as a character (chr), and in binary (bin), octal (oct), and hexadecimal (hex) format. Use the converted float to display the value rounded to no decimal places (round), and formatted as currency with two decimal places (format). Include try and except to handle input errors. Test the program with both integer and floating point values.
  2. Create a Python program that asks the user how old they are in years. Then ask the user if they would like to know how old they are in months, days, hours, or seconds. Use a condition statement to determine their selection, and use functions to convert years to months, years to days, years to hours, and years to seconds. Display their approximate age in the selected timeframe. Include try and except to handle input errors.
  3. Review MathsIsFun: Conversion of Temperature. Create a Python program that asks the user if they would like to convert Fahrenheit to Celsius or Celsius to Fahrenheit. Use a condition statement to determine their selection and then gather the appropriate input. Use functions to convert Fahrenheit to Celsius and Celsius to Fahrenheit. Calculate and display the converted temperature. Include try and except to handle input errors.
  4. Review MathsIsFun: Area of Plane Shapes. Create a Python program that asks the user what shape they would like to calculate the area for. Use a condition statement to determine their selection and then gather the appropriate input. Use separate functions to calculate the area of each shape and then calculate and display the area of the selected shape. Include try and except to handle input errors.
  5. Review MathsIsFun: Greatest Common Factor. Create a Python program that asks the user to enter two integer values. Based on the recursive algorithm provided in Wikipedia: Recursion (computer science), use a recursive function to calculate the greatest common factor (greatest common divisor) of the two values and then display the result. Include try and except to handle input errors.

Lesson Summary

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Function Concepts

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  • A function or subroutine is a sequence of program instructions that perform a specific task, packaged as a unit.[23]
  • In different programming languages, a function may be called a procedure, a subroutine, a routine, a method, or a subprogram.[24]
  • A function is often coded so that it can be started (called) several times and/or from several places during one execution of the program, including from other functions, and then branch back (return) to the next instruction after the call once the function's task is done.[25]
  • The content of a function is its body, the piece of program code that is executed when the function is called or invoked.[26]
  • A function may be written so that it expects to obtain one or more data values from the calling program (its parameters or formal parameters). The calling program provides actual values for these parameters, called arguments.[27]
  • Function arguments may be passed using call-by-reference or call-by-value.[28]
  • A function may also return a computed value to its caller (its return value), or provide various result values or out(put) parameters.[29]
  • A function can be coded so that it may call itself recursively, at one or more places, to perform its task.[30]
  • The advantages of breaking a program into functions include:[31]
    • decomposing a complex programming task into simpler steps
    • reducing duplicate code within a program
    • enabling reuse of code across multiple programs
    • hiding implementation details from users of the function
  • With call by value, a parameter acts within the subroutine as a variable initialized to the value of the argument (a local (isolated) copy of the argument).[32]
  • With call by reference, the argument supplied by the caller can be affected by actions within the called subroutine.[33]
  • Some programming languages allow for a default argument to be explicitly or implicitly given in a subroutine's declaration.[34]
  • Some languages allow functions to be defined to accept a variable number of arguments. For such languages, the functions must iterate through the list of arguments.[35]
  • Some programming languages allow functions to have named parameters.[36]

Python Functions

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  • A Python function definition defines a user-defined function object. The basic syntax for declaring a function is:[37]
    def <name>([parameters]):
        <statements>
  • Python arguments are passed using call by value.[38]
  • By default, Python arguments are passed by position. Parameter names may be used to identify parameters, bypassing position.[39]
  • A recursive function definition has one or more base cases, meaning input(s) for which the function produces a result trivially (without recurring), and one or more recursive cases, meaning input(s) for which the program recurs (calls itself).[40]
  • The job of the recursive cases can be seen as breaking down complex inputs into simpler ones. In a properly designed recursive function, with each recursive call, the input problem must be simplified in such a way that eventually the base case must be reached.[41]
  • Recursive programming solutions include mathematics calculations, data structure searches, and file system processing.[42]
  • The Python interpreter has a number of functions and types built into it that are always available.[43]
  • A function definition defines a user-defined function object.[44] If specified, return leaves the current function call with the expression list (or None) as return value.[45]
  • If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.[46]
  • Variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.[47]
  • The nonlocal statement causes the listed identifiers to refer to previously bound variables in the nearest enclosing scope excluding globals.[48][49]
  • A parameter is a named entity in a function definition that specifies an argument that the function can accept.[50] Functions may specify a default value for one or more arguments.[51] Functions may also be called using keyword arguments rather than positional arguments.[52]
  • Errors should never pass silently.[53]
  • Parameter validation is automated processing to validate the spelling or accuracy of parameters passed to a function or module.[54]
  • An assertion is a statement that a true–false expression is expected to always be true at that point in the code.[55]
  • The assert statement verifies that an expression is true or it raises an AssertionError. Unlike conditions, which are always executed, assert statements may be disabled when when optimization is requested (using command line option -O).[56]
  • Functions may accept a variable number of arguments using the * operator.[57] The variable arguments are provided as a Python list.
  • Variable named arguments may be accessed using the ** operator.[58][59][60] The variable named arguments are provided as a Python dictionary.
  • A docstring is a string literal that occurs as the first statement in a module, function, class, or method definition. Such a docstring becomes the __doc__ special attribute of that object. For consistency, always use """triple double quotes""" around docstrings.[61][62]

Key Terms

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algorithm
A general process for solving a category of problems.[63]
argument
A value provided to a function when the function is called. This value is assigned to the corresponding parameter in the function.[64]
body
The sequence of statements inside a function definition.[65]
composition
Using an expression as part of a larger expression, or a statement as part of a larger statement.[66]
deterministic
Pertaining to a program that does the same thing each time it runs, given the same inputs.[67]
dot notation
The syntax for calling a function in another module by specifying the module name followed by a dot (period) and the function name.[68]
flow of execution
The order in which statements are executed during a program run.[69]
fruitful function
A function that returns a value.[70]
function
A named sequence of statements that performs some useful operation. Functions may or may not take arguments and may or may not produce a result.[71]
function call
A statement that executes a function. It consists of the function name followed by an argument list.[72]
function definition
A statement that creates a new function, specifying its name, parameters, and the statements it executes.[73]
function object
A value created by a function definition. The name of the function is a variable that refers to a function object.[74]
header
The first line of a function definition.[75]
import statement
A statement that reads a module file and creates a module object.[76]
module object
A value created by an import statement that provides access to the data and code defined in a module.[77]
parameter
A name used inside a function to refer to the value passed as an argument.[78]
pseudorandom
Pertaining to a sequence of numbers that appear to be random, but are generated by a deterministic program.[79]
return value
The result of a function. If a function call is used as an expression, the return value is the value of the expression.[80]
void function
A function that does not return a value.[81]

Review Questions

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Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. A function or subroutine is _____.
    A function or subroutine is a sequence of program instructions that perform a specific task, packaged as a unit.
  2. In different programming languages, a function may be called a _____, a _____, a _____, a _____, or a _____.
    In different programming languages, a function may be called a procedure, a subroutine, a routine, a method, or a subprogram.
  3. A function is often coded so that _____.
    A function is often coded so that it can be started (called) several times and/or from several places during one execution of the program, including from other functions, and then branch back (return) to the next instruction after the call once the function's task is done.
  4. The content of a function is _____.
    The content of a function is its body, the piece of program code that is executed when the function is called or invoked.
  5. A function may be written so that it expects to obtain one or more data values from _____.
    A function may be written so that it expects to obtain one or more data values from the calling program (its parameters or formal parameters). The calling program provides actual values for these parameters, called arguments.
  6. Function arguments may be passed using _____ or _____.
    Function arguments may be passed using call-by-reference or call-by-value.
  7. A function may also return _____.
    A function may also return a computed value to its caller (its return value), or provide various result values or out(put) parameters.
  8. A function can be coded so that it may call itself _____, at one or more places, to perform its task.
    A function can be coded so that it may call itself recursively, at one or more places, to perform its task.
  9. The advantages of breaking a program into functions include:
    The advantages of breaking a program into functions include:

    decomposing a complex programming task into simpler steps
    reducing duplicate code within a program
    enabling reuse of code across multiple programs
    hiding implementation details from users of the function

  10. With call by value, a parameter acts within the subroutine as _____.
    With call by value, a parameter acts within the subroutine as a variable initialized to the value of the argument (a local (isolated) copy of the argument).
  11. With call by reference, the argument supplied by the caller _____.
    With call by reference, the argument supplied by the caller can be affected by actions within the called subroutine.
  12. Some programming languages allow for a default argument to be _____.
    Some programming languages allow for a default argument to be explicitly or implicitly given in a subroutine's declaration.
  13. Some languages allow functions to be defined to accept a _____.
    Some languages allow functions to be defined to accept a variable number of arguments. For such languages, the functions must iterate through the list of arguments.
  14. Some programming languages allow functions to have _____ parameters.
    Some programming languages allow functions to have named parameters.
  15. A Python function definition defines a user-defined function object. The basic syntax for declaring a function is:
    A Python function definition defines a user-defined function object. The basic syntax for declaring a function is:

    def <name>([parameters]):
        <statements>

  16. Python arguments are passed using call by _____.
    Python arguments are passed using call by value.
  17. By default, Python arguments are passed by _____.
    By default, Python arguments are passed by position. Parameter names may be used to identify parameters, bypassing position.
  18. A recursive function definition has _____.
    A recursive function definition has one or more base cases, meaning input(s) for which the function produces a result trivially (without recurring), and one or more recursive cases, meaning input(s) for which the program recurs (calls itself).
  19. The job of the recursive cases can be seen as _____.
    The job of the recursive cases can be seen as breaking down complex inputs into simpler ones. In a properly designed recursive function, with each recursive call, the input problem must be simplified in such a way that eventually the base case must be reached.
  20. Recursive programming solutions include _____.
    Recursive programming solutions include mathematics calculations, data structure searches, and file system processing.
  21. The Python interpreter has a number of functions and types _____.
    The Python interpreter has a number of functions and types built into it that are always available.
  22. A function definition defines _____.
    A function definition defines a user-defined function object. If specified, return leaves the current function call with the expression list (or None) as return value.
  23. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be _____.
    If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.
  24. Variables that are only referenced inside a function are implicitly _____. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a _____.
    Variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.
  25. The nonlocal statement causes the listed identifiers to _____.
    The nonlocal statement causes the listed identifiers to refer to previously bound variables in the nearest enclosing scope excluding globals.
  26. A parameter is _____.
    A parameter is a named entity in a function definition that specifies an argument that the function can accept. Functions may specify a default value for one or more arguments. Functions may also be called using keyword arguments rather than positional arguments.
  27. Errors should _____.
    Errors should never pass silently.
  28. Parameter validation is _____.
    Parameter validation is automated processing to validate the spelling or accuracy of parameters passed to a function or module.
  29. An assertion is _____.
    An assertion is a statement that a true–false expression is expected to always be true at that point in the code.
  30. The assert statement _____.
    The assert statement verifies that an expression is true or it raises an AssertionError. Unlike conditions, which are always executed, assert statements may be disabled when when optimization is requested (using command line option -O).
  31. Functions may accept a variable number of arguments using _____.
    Functions may accept a variable number of arguments using the * operator. The variable arguments are provided as a Python list.
  32. Variable named arguments may be accessed using _____.
    Variable named arguments may be accessed using the ** operator. The variable named arguments are provided as a Python dictionary.
  33. A docstring is _____.
    A docstring is a string literal that occurs as the first statement in a module, function, class, or method definition. Such a docstring becomes the __doc__ special attribute of that object. For consistency, always use """triple double quotes""" around docstrings.

Assessments

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See Also

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References

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  1. Vskills: Certified Python Developer
  2. Python.org: Built-in Functions
  3. Python.org: Function definitions
  4. Python.org: The return statement
  5. Python.org: Rules for local and global variables
  6. Python.org: Rules for local and global variables
  7. Python.org: Nonlocal
  8. Python.org: Programming FAQ
  9. Python.org: Parameter
  10. Python.org: More flow control tools
  11. Python.org: More flow control tools
  12. Python.org: The Zen of Python
  13. Wikipedia: Parameter validation
  14. Wikipedia: Assertion (software development)
  15. Python.org: The assert statement
  16. Python.org: More Flow Control Tools
  17. Python.org: More Flow Control Tools
  18. PythonTips: args and kwargs explained
  19. TutorialsPoint: Python Dictionary
  20. Python.org: Docstring Conventions
  21. Python.org: Documentation Strings
  22. Wikipedia: Factorial
  23. Wikipedia: Subroutine
  24. Wikipedia: Subroutine
  25. Wikipedia: Subroutine
  26. Wikipedia: Subroutine
  27. Wikipedia: Subroutine
  28. Wikipedia: Subroutine
  29. Wikipedia: Subroutine
  30. Wikipedia: Subroutine
  31. Wikipedia: Subroutine
  32. Wikipedia: Parameter (computer programming)
  33. Wikipedia: Parameter (computer programming)
  34. Wikipedia: Parameter (computer programming)
  35. Wikipedia: Parameter (computer programming)
  36. Wikipedia: Parameter (computer programming)
  37. Python.org: Function definitions
  38. Python.org: Defining Functions
  39. Python.org: Keyword Arguments
  40. Wikipedia: Recursion (computer science)
  41. Wikipedia: Recursion (computer science)
  42. Wikipedia: Recursion (computer science)
  43. Python.org: Built-in Functions
  44. Python.org: Function definitions
  45. Python.org: The return statement
  46. Python.org: Rules for local and global variables
  47. Python.org: Rules for local and global variables
  48. Python.org: Nonlocal
  49. Python.org: Programming FAQ
  50. Python.org: Parameter
  51. Python.org: More flow control tools
  52. Python.org: More flow control tools
  53. Python.org: The Zen of Python
  54. Wikipedia: Parameter validation
  55. Wikipedia: Assertion (software development)
  56. Python.org: The assert statement
  57. Python.org: More Flow Control Tools
  58. Python.org: More Flow Control Tools
  59. PythonTips: args and kwargs explained
  60. TutorialsPoint: Python Dictionary
  61. Python.org: Docstring Conventions
  62. Python.org: Documentation Strings
  63. PythonLearn: Functions
  64. PythonLearn: Functions
  65. PythonLearn: Functions
  66. PythonLearn: Functions
  67. PythonLearn: Functions
  68. PythonLearn: Functions
  69. PythonLearn: Functions
  70. PythonLearn: Functions
  71. PythonLearn: Functions
  72. PythonLearn: Functions
  73. PythonLearn: Functions
  74. PythonLearn: Functions
  75. PythonLearn: Functions
  76. PythonLearn: Functions
  77. PythonLearn: Functions
  78. PythonLearn: Functions
  79. PythonLearn: Functions
  80. PythonLearn: Functions
  81. PythonLearn: Functions

Lesson 6 - Strings

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This lesson introduces Python string processing.


Objectives and Skills

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Objectives and skills for this lesson include:

  • Standard Library
    • String operations

Readings

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  1. Wikipedia: String (computer science)
  2. Python for Everyone: Strings

Multimedia

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  1. YouTube: Python for Informatics - Chapter 6 - Strings
  2. YouTube: Python - Strings
  3. YouTube: Python - String Slicing
  4. YouTube: Python - String Formatting
  5. YouTube: Python Strings Playlist

Examples

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len() function

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The len() function returns the length (the number of items) of an object.[1]

string = "Test"

print("len(string):", len(string))

Output:

len(string): 4

Strings

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Textual data in Python is handled with str objects, or strings. Strings are immutable sequences of Unicode code points.[2]

string = "Test"

print("Characters:")
for letter in string:
    print(letter)

print("\nCharacters by position:")
for i in range(len(string)):
    print("string[%d]: %c" % (i, string[i]))

Output:

Characters:
T
e
s
t

Characters by position:
string[0]: T
string[1]: e
string[2]: s
string[3]: t

Membership Comparisons

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The operators in and not in test for membership. x in s evaluates to true if x is a member of s, and false otherwise.[3]

alphabet = "abcdefghijklmnopqrstuvwxyz"
string = "Python Programming/Strings"

print("The string contains:")
for letter in alphabet:
    if letter in string.lower():
        print(letter)

Output:

The string contains:
a
g
h
i
m
n
o
p
r
s
t
y

String Methods

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Strings implement all of the common sequence operations, along with additional methods such as case validation and conversion.[4]

string = "Test"

print("string:", string)
print("string.isalpha():", string.isalpha())
print("string.islower():", string.islower())
print("string.isnumeric():", string.isnumeric())
print("string.isspace():", string.isspace())
print("string.istitle():", string.istitle())
print("string.isupper():", string.isupper())
print("string.lower():", string.lower())
print("string.strip():", string.strip())
print("string.swapcase():", string.swapcase())
print("string.title():", string.title())
print("string.upper():", string.upper())

Output:

string: Test
string.isalpha(): True
string.islower(): False
string.isnumeric(): False
string.isspace(): False
string.istitle(): True
string.isupper(): False
string.lower(): test
string.strip(): Test
string.swapcase(): tEST
string.title(): Test
string.upper(): TEST

String Parsing

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Python substrings are referred to as slices. Slices are accessed using the syntax string[start:end], with the first character index starting at zero. The slice will include the characters from start up to but not including end. If end is omitted, the slice will include the characters from start through the end of the string. String slices may use negative indexing, in which case the index counts backwards from the end of the string.[5] The find() method returns the lowest index in the string where a substring is found within the given slice. Returns -1 if the substring is not found.[6]

string = "Python Programming/Strings"
index = string.find("/")

if index >= 0:
    project = string[0:index]
    page = string[index + 1:]

    print("Project:", project)
    print("Page:", page)

Output:

Project: Python Programming
Page: Strings

String Formatting

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String objects have one unique built-in operation: the % operator (modulo). This is also known as the string formatting or interpolation operator. Given format % values (where format is a string), % conversion specifications in format are replaced with zero or more elements of values.[7]

print("Value:", value)
print("Integer: %i" % value)
print("Octal: %o" % value)
print("Hexadecimal: %x" % value)
print("Float: %.2f" % value)
print("Exponent: %.2e" % value)
print("Character: %c" % value)
print("String: %s" % value)
print("Multiple: %i, %o, %x, %.2f, %.2e, %c, %s" % 
    (value, value, value, value, value, value, value))

Output:

Value: 65.5
Integer: 65
Octal: 101
Hexadecimal: 41
Float: 65.50
Exponent: 6.55e+01
Character: A
String: 65.5
Multiple: 65, 101, 41, 65.50, 6.55e+01, A, 65.5

str.format() Method

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The str.format() method uses format strings that contain “replacement fields” surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output.[8]

integer = 65
float = 65.5
string = "65.5"

print("Decimal: {:d}".format(integer))
print("Binary: {:b}".format(integer))
print("Octal: {:o}".format(integer))
print("Hexadecimal: {:x}".format(integer))
print("Character: {:c}".format(integer))
print("Float: {:.2f}".format(float))
print("Exponent: {:.2e}".format(float))
print("String: {:s}".format(string))
print("Multiple: {:d}, {:b}, {:o}, {:x}, {:c}, {:.2f}, {:.2e}, {:s}".format(
    integer, integer, integer, integer, integer, float, float, string))

Output:

Decimal: 65
Binary: 1000001
Octal: 101
Hexadecimal: 41
Character: A
Float: 65.50
Exponent: 6.55e+01
String: 65.5
Multiple: 65, 1000001, 101, 41, A, 65.50, 6.55e+01, 65.5


String Interpolation

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Introduced in Python 3.6, string interpolation formats strings that contain “replacement fields” surrounded by curly braces {}, similar to the str.format() method.[9] However, an uppercase or lowercase 'f' is placed before the string to indicate string formatting:

integer = 65
float = 65.5
string = "65.5"

print(f"Decimal: {integer:d}")
print(f"Binary: {integer:b}")
print(f"Octal: {integer:o}")
print(f"Hexadecimal: {integer:x}")
print(f"Character: {integer:c}")
print(f"Float: {float:.2f}")
print(f"Exponent: {float:.2e}")
print(f"String: {string:s}")
print(f"Multiple: {integer:d}, {integer:b}, {integer:o}, {integer:x}, {integer:c}, {float:.2f}, {float:.2e}, {string:s}")

Output:

Decimal: 65
Binary: 1000001
Octal: 101
Hexadecimal: 41
Character: A
Float: 65.50
Exponent: 6.55e+01
String: 65.5
Multiple: 65, 1000001, 101, 41, A, 65.50, 6.55e+01, 65.5

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Practice

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  1. Review Python.org: String methods. Create a Python program that asks the user for a line of text containing a first name and last name. Use string methods to parse the line and print out the name in the form last name, first initial, such as Lastname, F. Include a trailing period after the first initial. Ensure that the first letter of each name part is capitalized, and the rest of the last name is lower case. Include error handling in case the user does not enter exactly two name parts. Use a user-defined function for the actual string processing, separate from input and output.
  2. Review Python.org: String methods. Create a Python program that asks the user for a line of comma-separated-values. It could be a sequence of test scores, names, or any other values. Use string methods to parse the line and print out each item on a separate line. Remove commas and any leading or trailing spaces from each item when printed. If the item is numeric, display it formatted as a floating point value with two decimal places. If the value is not numeric, display it as is.
  3. Review Python.org: String methods. Create a Python program that asks the user for a line of text that contains HTML tags, such as:
        <p><strong>This is a bold paragraph.</strong></p>
    Use string methods to search for and remove all HTML tags, and then print the remaining untagged text. Include error handling in case an HTML tag isn't entered correctly (an unmatched < or >). Use a user-defined function for the actual string processing, separate from input and output.
  4. Review Python.org: String methods. Create a Python program that asks the user for a line of text. Then ask the user for the number of characters to print in each line, the number of lines to be printed, and a scroll direction, right or left. Using the given line of text, duplicate the text as needed to fill the given number of characters per line. Then print the requested number of lines, shifting the entire line's content by one character, left or right, each time the line is printed. The first or last character will be shifted / appended to the other end of the string. For example:
        Repeat this. Repeat this. 
        epeat this. Repeat this. R
        peat this. Repeat this. Re

Lesson Summary

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  • A string is traditionally a sequence of characters, either as a literal constant or as some kind of variable.[10]
  • A string variable may allow its elements to be mutated and the length changed, or it may be fixed (after creation).[11]
  • Python strings are immutable — they cannot be changed.[12]
  • The len() function returns the length (the number of items) of an object.[13]
  • Textual data in Python is handled with str objects, or strings. Strings are immutable sequences of Unicode code points.[14]
  • The operators in and not in test for membership. x in s evaluates to true if x is a member of s, and false otherwise.[15]
  • String methods include: isalpha(), islower(), isnumeric(), isspace(), istitle(), isupper(), lower(), strip(), swapcase(), title(), and upper().[16]
  • Python substrings are referred to as slices. Slices are accessed using the syntax string[start:end], with the first character index starting at zero. The slice will include the characters from start up to but not including end. If end is omitted, the slice will include the characters from start through the end of the string.[17]
  • String slices may use negative indexing, in which case the index counts backwards from the end of the string.[18]
  • The find() method returns the lowest index in the string where a substring is found within the given slice. Returns -1 if the substring is not found.[19]
  • String objects have one unique built-in operation: the % operator (modulo). This is also known as the string formatting or interpolation operator. Given format % values (where format is a string), % conversion specifications in format are replaced with zero or more elements of values.[20]
  • The str.format() method uses format strings that contain “replacement fields” surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output.[21]

Key Terms

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counter
A variable used to count something, usually initialized to zero and then incremented.[22]
empty string
A string with no characters and length 0, represented by two quotation marks.[23]
format operator
An operator, %, that takes a format string and a tuple and generates a string that includes the elements of the tuple formatted as specified by the format string.[24]
format sequence
A sequence of characters in a format string, like %d, that specifies how a value should be formatted.[25]
format string
A string, used with the format operator, that contains format sequences.[26]
flag
A boolean variable used to indicate whether a condition is true.[27]
invocation
A statement that calls a method.[28]
immutable
The property of a sequence whose items cannot be assigned.[29]
index
An integer value used to select an item in a sequence, such as a character in a string.[30]
item
One of the values in a sequence.[31]
method
A function that is associated with an object and called using dot notation.[32]
object
Something a variable can refer to. For now, you can use “object” and “value” interchangeably.[33]
search
A pattern of traversal that stops when it finds what it is looking for.[34]
sequence
An ordered set; that is, a set of values where each value is identified by an integer index.[35]
slice
A part of a string specified by a range of indices.[36]
traverse
To iterate through the items in a sequence, performing a similar operation on each.[37]

Review Questions

edit
Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. A string is _____.
    A string is traditionally a sequence of characters, either as a literal constant or as some kind of variable.
  2. A string variable may _____.
    A string variable may allow its elements to be mutated and the length changed, or it may be fixed (after creation).
  3. Python strings are _____ — they cannot be changed.
    Python strings are immutable — they cannot be changed.
  4. The len() function returns _____.
    The len() function returns the length (the number of items) of an object.
  5. Textual data in Python is handled with _____.
    Textual data in Python is handled with str objects, or strings. Strings are immutable sequences of Unicode code points.
  6. The operators _____ and _____ test for membership. x _____ s evaluates to true if x is a member of s, and false otherwise.
    The operators in and not in test for membership. x in s evaluates to true if x is a member of s, and false otherwise.
  7. String methods include:
    String methods include: isalpha(), islower(), isnumeric(), isspace(), istitle(), isupper(), lower(), strip(), swapcase(), title(), and upper().
  8. Python substrings are referred to as _____.
    Python substrings are referred to as slices.
  9. Slices are accessed using the syntax _____.
    Slices are accessed using the syntax string[start:end].
  10. The first character index in a slice starts at _____. The slice will include _____. If end is omitted, the slice will include _____.
    The first character index in a slice starts at zero. The slice will include the characters from start up to but not including end. If end is omitted, the slice will include the characters from start through the end of the string.
  11. String slices may use negative indexing, in which case _____.
    String slices may use negative indexing, in which case the index counts backwards from the end of the string.
  12. The find() method returns _____.
    The find() method returns the lowest index in the string where a substring is found within the given slice, and returns -1 if the substring is not found.
  13. String objects have one unique built-in operation: the % operator (modulo). This is also known as _____.
    String objects have one unique built-in operation: the % operator (modulo). This is also known as the string formatting or interpolation operator. Given format % values (where format is a string), % conversion specifications in format are replaced with zero or more elements of values.
  14. The str.format() method uses format strings that contain “replacement fields” surrounded by _____.
    The str.format() method uses format strings that contain “replacement fields” surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output.

Assessments

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See Also

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References

edit
  1. Python.org Built-in Functions
  2. Python.org: Built-in Types
  3. Python.org: Expressions
  4. Python.org: Built-in Types
  5. Python.org: Strings
  6. Python.org: Built-in Types
  7. Python.org: printf-style String Formatting
  8. Python.org: Format String Syntax
  9. Python, Real. "Python 3's f-Strings: An Improved String Formatting Syntax (Guide) – Real Python". realpython.com. Retrieved 2019-02-22.
  10. Wikipedia: String (computer science)
  11. Wikipedia: String (computer science)
  12. Python.org: Strings
  13. Python.org Built-in Functions
  14. Python.org: Built-in Types
  15. Python.org: Expressions
  16. Python.org: Built-in Types
  17. Python.org: Strings
  18. Python.org: Strings
  19. Python.org: Built-in Types
  20. Python.org: printf-style String Formatting
  21. Python.org: Format String Syntax
  22. PythonLearn: Strings
  23. PythonLearn: Strings
  24. PythonLearn: Strings
  25. PythonLearn: Strings
  26. PythonLearn: Strings
  27. PythonLearn: Strings
  28. PythonLearn: Strings
  29. PythonLearn: Strings
  30. PythonLearn: Strings
  31. PythonLearn: Strings
  32. PythonLearn: Strings
  33. PythonLearn: Strings
  34. PythonLearn: Strings
  35. PythonLearn: Strings
  36. PythonLearn: Strings
  37. PythonLearn: Strings

Lesson 7 - Lists

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This lesson introduces Python lists.


Objectives and Skills

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Objectives and skills for this lesson include:[1]

  • Data Structures
    • List

Readings

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  1. Wikipedia: List (computer science)
  2. Python for Everyone: Lists

Multimedia

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  1. YouTube: Python for Informatics - Chapter 8 - Lists
  2. YouTube: Python Lists
  3. YouTube: Immutable vs Mutable Objects in Python

Examples

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Lists

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Python supports a number of compound data types used to group other values. The most versatile is the list, which can be written as a list of comma-separated values (items) between square brackets. Lists might contain items of different types, but usually the items all have the same type.[2]

list = [1, 2, 3]

for item in list:
    print(item)

Output:

1
2
3

Index

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List items can be accessed by index.[3]

list = [1, 2, 3]

for i in range(len(list)):
    print("list[%d]: %d" % (i, list[i]))

Output:

list[0]: 1
list[1]: 2
list[2]: 3

Concatenation

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Lists support concatenation.[4]

list = [1, 2, 3]

list += [4, 5]
for i in range(len(list)):
    print("list[%d]: %d" % (i, list[i]))

Output:

list[0]: 1
list[1]: 2
list[2]: 3
list[3]: 4
list[4]: 5

Slicing

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Lists may be sliced.[5]

list = [1, 2, 3, 4, 5]

list = list[0:2] + list[3:]
for i in range(len(list)):
    print("list[%d]: %d" % (i, list[i]))

Output:

list[0]: 1
list[1]: 2
list[2]: 4
list[3]: 5

Mutable

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List items are mutable, i.e. it is possible to change their content.[6]

list = [1, 2, 3]

for i in range(len(list)):
    list[i] = -list[i]
    print("list[%d]: %d" % (i, list[i]))

Output:

list[0]: -1
list[1]: -2
list[2]: -3

Multidimensional

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Multi dimensional lists are lists within lists.[7]

table = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

for row in range(len(table)):
    for column in range(len(table[row])):
        print(table[row][column], end = '\t')
    print()

Output:

1       2       3
4       5       6
7       8       9

Methods

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Lists support a variety of methods, including:[8]

  • list.append(x) - Adds an item to the end of the list.
  • list.insert(i, x) - Inserts an item at a given position.
  • list.remove(x) - Removes the first item from the list whose value is x
  • list.reverse() - Reverse the elements of the list in place.
  • list.sort() - Sorts the items of the list in place.
  • list.clear() - Removes all items from the list.
list = []

print("List:", list)

list.append(1)
list.append(2)
print("After append:", list)

list.insert(2, 3)
list.insert(0, 4)
print("After insert:", list)

list.remove(4)
print("After remove:", list)

list.reverse()
print("After reverse:", list)

list.sort()
print("After sort:", list)

list.clear()
print("After clear:", list)

Output:

List: []
After append: [1, 2]
After insert: [4, 1, 2, 3]
After remove: [1, 2, 3]
After reverse: [3, 2, 1]
After sort: [1, 2, 3]
After clear: []

Stacks and Queues

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The list.pop([i]) method removes the item at the given position in the list, and returns it. If no index is specified, pop() removes and returns the last item in the list.[9] This allows for simple stack (last-in, first-out) and queue (first-in, first-out) processing.

print("Stack: Last in, first out")
list = []
list.append(1)
list.append(2)
list.append(3)
while len(list) > 0:
    print(list.pop())

print("\nQueue: First in, first out")
list = []
list.append(1)
list.append(2)
list.append(3)
while len(list) > 0:
    print(list.pop(0))

Output:

Stack: Last in, first out
3
2
1

Queue: First in, first out
1
2
3

Split

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The string split(<separator>) method returns a list of the words in the string, using separator as the delimiter string. If separator is not specified, runs of consecutive whitespace are regarded as a single separator.[10]

string = "This is a test"
list = string.split()
print(list)

string = "So-is-this"
list = string.split("-")
print(list)

Output:

['This', 'is', 'a', 'test']
['So', 'is', 'this']

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Practice

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  1. Create a Python program that asks the user to enter grade scores. Start by asking the user how many scores they would like to enter. Then use a loop to request each score and add it to a list. Finally, display the list of entered scores sorted in descending order and then calculate and display the high, low, and average for the entered scores. Include try and except to handle input errors.
  2. Create a Python program that asks the user to enter a line of comma-separated grade scores. Use the string split() method to parse the line and display the list of entered scores sorted in descending order and then calculate and display the high, low, and average for the entered scores. Include try and except to handle input errors.
  3. Create a Python program that asks the user to enter a line of text. Use the string split() method to parse the line and then display the list of words in alphabetical order with any duplicate words removed from the list.
  4. Create a Python program that asks the user for a line of text that contains HTML tags, such as:
        <p><strong>This is a bold paragraph.</strong></p>
    Use string methods to search for and remove all HTML tags from the text, saving each removed tag in a list. Print the untagged text and then display the list of removed tags sorted in alphabetical order with duplicate tags removed. Include error handling in case an HTML tag isn't entered correctly (an unmatched < or >). Use a user-defined function for the actual string processing, separate from input and output.

Lesson Summary

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List Concepts

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  • A list or sequence is a data type that represents an ordered sequence of values, where the same value may occur more than once.[11]
  • Lists are a basic example of containers, as they contain other values. If the same value occurs multiple times, each occurrence is considered a distinct item.[12]
  • A list can often be constructed by writing the items in sequence, separated by commas, semicolons, or spaces, within a pair of delimiters such as parentheses '()', brackets '[]', braces '{}', or angle brackets '<>'. [13]
  • Some languages may allow list types to be indexed or sliced like array types, in which case the data type is more accurately described as an array.[14]
  • An array is a data structure consisting of a collection of elements (values or variables), each identified by at least one array index or key.[15]
  • Lists can be used to store a list of elements. However, unlike in traditional arrays, lists can expand and shrink, and are stored dynamically in memory.[16]

Python Lists

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  • Python lists can be written as a list of comma-separated values (items) between square brackets.[17]
  • Lists might contain items of different types, but usually the items all have the same type.[18]
  • List items can be accessed by index using the syntax list[index].[19]
  • Lists support concatenation using the syntax list + list.[20]
  • Lists may be sliced using the syntax list[start:end]. The slice will include the starting index value up to but not including the ending index value.[21]
  • List items are mutable, i.e. it is possible to change their content.[22]
  • Multidimensional lists are lists within lists, accessed using the syntax list[index][index].[23]
  • list.append(x) - adds an item to the end of the list.[24]
  • list.insert(i, x) - Inserts an item at a given position.[25]
  • list.remove(x) - Removes the first item from the list whose value is x.[26]
  • list.reverse() - Reverse the elements of the list in place.[27]
  • list.sort() - Sorts the items of the list in place.[28]
  • list.clear() - Removes all items from the list.[29]
  • list.pop([i]) removes the item at the given position in the list, and returns it. If no index is specified, pop() removes and returns the last item in the list.[30]
  • string.split(<separator>) returns a list of the words in the string, using separator as the delimiter string. If separator is not specified, runs of consecutive whitespace are regarded as a single separator.[31]

Key Terms

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aliasing
A circumstance where two or more variables refer to the same object.[32]
delimiter
A character or string used to indicate where a string should be split.[33]
element
One of the values in a list (or other sequence); also called items.[34]
equivalent
Having the same value.[35]
index
An integer value that indicates an element in a list.[36]
identical
Being the same object (which implies equivalence).[37]
list
A sequence of values.[38]
list traversal
The sequential accessing of each element in a list.[39]
nested list
A list that is an element of another list.[40]
object
Something a variable can refer to. An object has a type and a value.[41]
queue
A data structure in which the entities in the collection are kept in order and the principal (or only) operations on the collection are the addition of entities to the end of the collection and removal of entities from the start of the collection (first-in, first-out).[42]
reference
The association between a variable and its value.[43]
stack
A data structure in which the entities in the collection are kept in order and the principal (or only) operations on the collection are push, which adds an element to the collection, and pop, which removes the most recently added element that was not yet removed (last-in, first-out).[44]

Review Questions

edit
Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. A list or sequence is _____.
    A list or sequence is a data type that represents an ordered sequence of values, where the same value may occur more than once.
  2. Lists are a basic example of _____. If the same value occurs multiple times, _____.
    Lists are a basic example of containers, as they contain other values. If the same value occurs multiple times, each occurrence is considered a distinct item.
  3. A list can often be constructed by _____. 
    A list can often be constructed by writing the items in sequence, separated by commas, semicolons, or spaces, within a pair of delimiters such as parentheses '()', brackets '[]', braces '{}', or angle brackets '<>'. 
  4. Some languages may allow list types to be _____ like array types, in which case the data type is more accurately described as an array.
    Some languages may allow list types to be indexed or sliced like array types, in which case the data type is more accurately described as an array.
  5. An array is _____.
    An array is a data structure consisting of a collection of elements (values or variables), each identified by at least one array index or key.
  6. Lists can be used to store a list of elements. However, unlike in traditional arrays, lists can _____, and are stored _____.
    Lists can be used to store a list of elements. However, unlike in traditional arrays, lists can expand and shrink, and are stored dynamically in memory.
  7. Python lists can be written as _____.
    Python lists can be written as a list of comma-separated values (items) between square brackets.
  8. Lists might contain _____.
    Lists might contain items of different types, but usually the items all have the same type.
  9. List items can be accessed by index using the syntax _____.
    List items can be accessed by index using the syntax list[index].
  10. Lists support concatenation using the syntax _____.
    Lists support concatenation using the syntax list + list.
  11. Lists may be sliced using the syntax _____.
    Lists may be sliced using the syntax list[start:end]. The slice will include the starting index value up to but not including the ending index value.
  12. List items are mutable, i.e. _____.
    List items are mutable, i.e. it is possible to change their content.
  13. Multidimensional lists are _____.
    Multidimensional lists are lists within lists, accessed using the syntax list[index][index].
  14. list.append(x) - _____.
    list.append(x) - Adds an item to the end of the list.
  15. list.insert(i, x) - _____.
    list.insert(i, x) - Inserts an item at a given position.
  16. list.remove(x) - _____.
    list.remove(x) - Removes the first item from the list whose value is x.
  17. list.reverse() - _____.
    list.reverse() - Reverse the elements of the list in place.
  18. list.sort() - _____.
    list.sort() - Sorts the items of the list in place.
  19. list.clear() - _____.
    list.clear() - Removes all items from the list.
  20. list.pop() - _____.
    list.pop() - Removes the item at the given position in the list, and returns it. If no index is specified, pop() removes and returns the last item in the list.
  21. string.split() - _____.
    string.split() - Returns a list of the words in the string, using separator as the delimiter string. If separator is not specified, runs of consecutive whitespace are regarded as a single separator.

Assessments

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See Also

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References

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Lesson 8 - Dictionaries

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This lesson introduces Python dictionaries.


Objectives and Skills

edit

Objectives and skills for this lesson include:[1]

  • Data Structures
    • Dictionary

Readings

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  1. Wikipedia: Dictionary (data structure)
  2. Python for Everyone: Dictionaries

Multimedia

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  1. YouTube: Python for Informatics - Chapter 9 - Dictionaries
  2. YouTube: Python Dictionary
  3. YouTube: Python - Min, Max, and Sorting Dictionaries
  4. YouTube: Python Dictionaries Playlist

Examples

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Dictionaries

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Dictionaries are collections that are indexed by keys, which can be any immutable type. It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary). Placing a comma-separated list of key:value pairs within braces adds initial key:value pairs to the dictionary.[2]

dictionary = {
    'cat': 'Frisky',
    'dog': 'Spot',
    'fish': 'Bubbles',
}

print("dictionary:", dictionary)
print("dictionary['cat']:", dictionary['cat'])
print("dictionary['dog']:", dictionary['dog'])
print("dictionary['fish']:", dictionary['fish'])

Output:

dictionary: {'dog': 'Spot', 'fish': 'Bubbles', 'cat': 'Frisky'}
dictionary['cat']: Frisky
dictionary['dog']: Spot
dictionary['fish']: Bubbles

Dictionary Updates

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A pair of braces creates an empty dictionary: {}. Dictionary items may be added, updated, and deleted by key.[3]

dictionary = {}
dictionary['cat'] = 'Frisky'
dictionary['dog'] = 'Spot'
dictionary['fish'] = 'Bubbles'
print("dictionary:", dictionary)

dictionary['dog'] = 'Rover'
print("dictionary:", dictionary)

del dictionary['fish']
print("dictionary:", dictionary)

Output:

dictionary: {'cat': 'Frisky', 'fish': 'Bubbles', 'dog': 'Spot'}
dictionary: {'cat': 'Frisky', 'fish': 'Bubbles', 'dog': 'Rover'}
dictionary: {'cat': 'Frisky', 'dog': 'Rover'}

Multidimensional Dictionaries

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Dictionaries may be nested.

dictionary = {
    'John': {'name': 'John Smith', 'phone': '123-555-0101'}, 
    'Jose': {'name': 'Jose Garcia', 'phone': '123-555-0102'}
}

print("John's name:", dictionary['John']['name'])
print("Jose's phone:",dictionary['Jose']['phone'])

Output:

John's name: John Smith
Jose's phone: 123-555-0102

Looping

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Dictionary items may be accessed with a for loop.

dictionary = {
    'cat': 'Frisky',
    'dog': 'Spot',
    'fish': 'Bubbles',
}

for key in dictionary:
    print("dictionary[%s]: %s" % (key, dictionary[key]))

Output:

dictionary[dog]: Spot
dictionary[fish]: Bubbles
dictionary[cat]: Frisky

Sorting by Key

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Dictionaries are unordered. Dictionary items may be displayed in key order by sorting a list of keys using the sorted() function and the dictionary.keys() method.[4]

dictionary = {
    'cat': 'Frisky',
    'dog': 'Spot',
    'fish': 'Bubbles',
}

for key in sorted(dictionary.keys()):
    print("dictionary[%s]: %s" % (key, dictionary[key]))

Output:

dictionary[cat]: Frisky
dictionary[dog]: Spot
dictionary[fish]: Bubbles

Sorting by Value

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Dictionaries are unordered. Dictionary items may be displayed in value order by sorting a list of key-value pairs using the sorted() function and the dictionary.get() method.[5]

dictionary = {
    'cat': 'Frisky',
    'dog': 'Spot',
    'fish': 'Bubbles',
}

for key in sorted(dictionary, key=dictionary.get):
    print("dictionary[%s]: %s" % (key, dictionary[key]))

Output:

dictionary[fish]: Bubbles
dictionary[cat]: Frisky
dictionary[dog]: Spot

Dictionary as Return Value

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Functions may return multiple values using a dictionary.[6]

def function(x, y):
    ...
    return {'x': x, 'y': y}

Activities

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Tutorials

edit
  1. Complete one or more of the following tutorials:

Practice

edit
  1. Create a Python program that asks the user to enter grade scores. Start by asking the user how many scores they would like to enter. Then use a loop to request each score and add it to a dictionary, counting how many times each score is entered. Finally, use a function to display the dictionary of entered scores sorted in descending order and a histogram showing how many times each score was entered. Include try and except to handle input errors. For example:
        100: *
        90: ***
        80: *****
        70: **
  2. Create a Python program that asks the user to enter a line of comma-separated grade scores. Use the string split() method to parse the line and add each score to a dictionary, counting how many times each score is entered. Finally, use a function to display the list of entered scores sorted in descending order and a histogram showing how many times each score was entered. Include try and except to handle input errors. For example:
        100: *
        90: ***
        80: *****
        70: **
  3. Create a Python program that asks the user to enter a line of text. Use the string split() method to parse the line and add each word to a dictionary, counting how many times each word is entered. Convert words to lower case and remove any punctuation to prevent duplicate or invalid results. Finally, use a function to display the list of entered words sorted in alphabetical order and a histogram showing how many times each word was entered. Include try and except to handle input errors. For example:
        cat: *
        hat: *
        in: *
        the: **
  4. Create a Python program that asks the user for a line of text that contains HTML tags, such as:
        <p><strong>This is a bold paragraph.</strong></p>
    Use string methods to search for and remove all HTML tags from the text, saving each removed tag in a dictionary. Print the untagged text and then use a function to display the list of removed tags sorted in alphabetical order and a histogram showing how many times each tag was used. Include error handling in case an HTML tag isn't entered correctly (an unmatched < or >). Use a user-defined function for the actual string processing, separate from input and output. For example:
        </p>: *
        </strong>: *
        <p>: *
        <strong>: *

Lesson Summary

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Dictionary Concepts

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  • A dictionary or associative array is a data type composed of a collection of (key: value) pairs, such that each possible key appears at most once in the collection.[7]
  • Dictionary operations include:[8]
    • the addition of a pair to the collection
    • the removal of a pair from the collection
    • the modification of an existing pair
    • the lookup of a value associated with a particular key
  • Dictionaries are typically implemented in programming languages as either a hash table or a search tree.[9]
  • A hash table uses a hash function to compute an index into an array of buckets or slots, from which the desired value can be found.[10]
  • A hash function is any function that can be used to map data of arbitrary size to data of fixed size.[11]
  • A good hash function should map the expected inputs as evenly as possible over its output range.[12]

Python Dictionaries

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  • Dictionaries are collections that are indexed by keys, which can be any immutable type.[13]
  • It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary).[14]
  • Placing a comma-separated list of key:value pairs within braces adds initial key:value pairs to the dictionary using the syntax dictionary = {key: value, key: value}.[15]
  • A pair of braces creates an empty dictionary: {}. Dictionary items may be added, updated, and deleted by key using the syntax dictionary[key] = value and del dictionary[key].[16]
  • Dictionaries may be nested and are accessed using the syntax dictionary[key][subkey]
  • Dictionary items may be accessed with a for loop.
  • Dictionaries are unordered. Dictionary items may be displayed in key order by sorting a list of keys using the sorted() function and the dictionary.keys() method.[17]
  • Dictionaries are unordered. Dictionary items may be displayed in value order by sorting a list of key-value pairs using the sorted() function and the dictionary.get() method.[18]

Key Terms

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dictionary
A mapping from a set of keys to their corresponding values.[19]
hashtable
The algorithm used to implement Python dictionaries.[20]
hash function
A function used by a hashtable to compute the location for a key.[21]
histogram
A set of counters.[22]
implementation
A way of performing a computation.[23]
item
Another name for a key-value pair.[24]
key
An object that appears in a dictionary as the first part of a key-value pair.[25]
key-value pair
The representation of the mapping from a key to a value.[26]
lookup
A dictionary operation that takes a key and finds the corresponding value.[27]
nested loops
When there are one or more loops “inside” of another loop. The inner loop runs to completion each time the outer loop runs once.[28]
value
An object that appears in a dictionary as the second part of a key-value pair. This is more specific than our previous use of the word “value”.[29]

Review Questions

edit
Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. A dictionary or associative array is _____.
    A dictionary or associative array is a data type composed of a collection of (key: value) pairs, such that each possible key appears at most once in the collection.
  2. Dictionary operations include:
    Dictionary operations include:
    the addition of a pair to the collection
    the removal of a pair from the collection
    the modification of an existing pair
    the lookup of a value associated with a particular key
  3. Dictionaries are typically implemented in programming languages as _____.
    Dictionaries are typically implemented in programming languages as either a hash table or a search tree.
  4. A hash table uses _____.
    A hash table uses a hash function to compute an index into an array of buckets or slots, from which the desired value can be found.
  5. A hash function is _____.
    A hash function is any function that can be used to map data of arbitrary size to data of fixed size.
  6. A good hash function should _____.
    A good hash function should map the expected inputs as evenly as possible over its output range.
  7. Python dictionaries are _____.
    Python dictionaries are collections that are indexed by keys, which can be any immutable type.
  8. It is best to think of a dictionary as _____.
    It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary).
  9. Placing a comma-separated list of key:value pairs within braces adds initial key:value pairs to the dictionary using the syntax _____.
    {{{2}}}
  10. A pair of braces creates an empty dictionary: {}. Dictionary items may be added, updated, and deleted by key using the syntax _____.
    {{{2}}}
  11. Dictionaries may be nested and are accessed using the syntax _____
    Dictionaries may be nested and are accessed using the syntax dictionary[key][subkey]
  12. Dictionary items may be accessed with a _____ loop.
    Dictionary items may be accessed with a for loop.
  13. Dictionaries are unordered. Dictionary items may be displayed in key order by _____.
    Dictionaries are unordered. Dictionary items may be displayed in key order by sorting a list of keys using the sorted() function and the dictionary.keys()method.
  14. Dictionaries are unordered. Dictionary items may be displayed in value order by _____.
    Dictionaries are unordered. Dictionary items may be displayed in value order by sorting a list of key-value pairs using the sorted() function and the dictionary.get() method.

Assessments

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See Also

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References

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Lesson 9 - Tuples and Sets

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This lesson introduces Python tuples and sets by comparing them to lists and dictionaries.


Objectives and Skills

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Objectives and skills for this lesson include:[1]

  • Data Structures
    • Tuple
    • Sequences
    • Set

Readings

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  1. Wikipedia: Data structure
  2. Python for Everyone: Tuples

Multimedia

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  1. YouTube: Python for Informatics - Chapter 10 - Tuples

Examples

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Data Structures

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Python data structures include lists, dictionaries, tuples, and sets.[2]

list = [1, 2, 3]
dictionary = {1: "one", 2: "two", 3: "three"}
tuple = (1, 2, 3)
set = {1, 2, 3}

print("list:", list)
print("dictionary:", dictionary)
print("tuple:", tuple)
print("set:", set)

Output:

list: [1, 2, 3]
dictionary: {1: 'one', 2: 'two', 3: 'three'}
tuple: (1, 2, 3)
set: {1, 2, 3}

Data Types

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The data types for a list, dictionary, tuple, and set, are 'list', 'dict', 'tuple', and 'set', respectively.[3]

list = [1, 2, 3]
dictionary = {1: "one", 2: "two", 3: "three"}
tuple = (1, 2, 3)
set = {1, 2, 3}

print("list:", type(list))
print("dictionary:", type(dictionary))
print("tuple:", type(tuple))
print("set:", type(set))

Output:

list: <class 'list'>
dictionary: <class 'dict'>
tuple: <class 'tuple'>
set: <class 'set'>

Functions

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The list(), dict(), tuple(), and set() functions may be used to create list, dictionary, tuple, and set objects, respectively.[4]

print(list((1, 2, 3)))
print(dict(one = 1, two = 2, three = 3))
print(tuple((1, 2, 3)))
print(set((1, 2, 3)))

Output:

[1, 2, 3]
{'three': 3, 'two': 2, 'one': 1}
(1, 2, 3)
{1, 2, 3}

Mutability

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List, dictionary and set items are mutable. Tuple items are immutable.[5]

list = [1, 2, 3]
dictionary = {1: "one", 2: "two", 3: "three"}
tuple = (1, 2, 3)
set = {1, 2, 3}

list[0] = 0
print("list: mutable")

dictionary[1] = "zero"
print("dictionary: mutable")

try:
    tuple[0] = 0
    print("tuple: mutable")
except:
    print("tuple: immutable")

try:
    set |= {0}
    print("set: mutable")
except:
    print("set: immutable")

print()
print("list:", list)
print("dictionary:", dictionary)
print("tuple:", tuple)
print("set:", set)

Output:

list: mutable
dictionary: mutable
tuple: immutable
set: mutable

list: [0, 2, 3]
dictionary: {1: 'zero', 2: 'two', 3: 'three'}
tuple: (1, 2, 3)
set: {0, 1, 2, 3}

Order

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Lists and tuples maintain order. Dictionaries and sets are unordered.[6]

list = ["fish", "dog", "cat"]
tuple = ("fish", "dog", "cat")
dictionary = {
    'fish': 'Bubbles',
    'dog': 'Spot',
    'cat': 'Frisky',
}
set = {"fish", "dog", "cat"}

print("list:", list)
print("dictionary:", dictionary)
print("tuple:", tuple)
print("set:", set)

Output:

list: ['fish', 'dog', 'cat']
tuple: ('fish', 'dog', 'cat')
dictionary: {'dog': 'Spot', 'fish': 'Bubbles', 'cat': 'Frisky'}
set: {'dog', 'fish', 'cat'}

Duplication

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Lists and tuples allow duplication. Dictionary and set items are unique.[7]

list = [1, 1, 1]
tuple = (1, 1, 1)
dictionary = {1: 'one', 1: 'one', 1: 'one'}
set = {1, 1, 1}

print("list:", list)
print("tuple:", tuple)
print("dictionary:", dictionary)
print("set:", set)

Output:

list: [1, 1, 1]
tuple: (1, 1, 1)
dictionary: {1: 'one'}
set: {1}

Sets may be used to remove duplication found in other data structures.[8]

list = [1, 1, 1]
tuple = (1, 1, 1)

print("list:", list)
print("set:", set(list))

print("tuple:", tuple)
print("set:", set(tuple))

Output:

list: [1, 1, 1]
set: {1}
tuple: (1, 1, 1)
set: {1}

Looping

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List, dictionary, tuple, and set items may be accessed using a for loop.[9]

list = ["fish", "dog", "cat"]
tuple = ("fish", "dog", "cat")
dictionary = {
    'fish': 'Bubbles',
    'dog': 'Spot',
    'cat': 'Frisky',
}
set = {"fish", "dog", "cat"}

print("List:")
for item in list:
    print(item)
print()

print("Dictionary:")
for item in dictionary:
    print(item)
print()

print("Tuple:")
for item in tuple:
    print(item)
print()

print("Set:")
for item in set:
    print(item)

Output:

List:
fish
dog
cat

Dictionary:
cat
fish
dog

Tuple:
fish
dog
cat

Set:
cat
fish
dog

Index

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List and tuple items may be accessed by index. Dictionary items are accessed by key. Set items cannot be accessed by index.[10]

list = [1, 2, 3]
dictionary = {1: "one", 2: "two", 3: "three"}
tuple = (1, 2, 3)
set = {1, 2, 3}

for i in range(len(list)):
    print("list[%d]: %d" % (i, list[i]))
print()

for key in dictionary:
    print("dictionary[%s]: %s" % (key, dictionary[key]))
print()

for i in range(len(tuple)):
    print("tuple[%d]: %d" % (i, tuple[i]))
print()

try:
    for i in range(len(set)):
        print("set[%d]: %d" % (i, set[i]))
except Exception as exception:
    print(exception)

Output:

list[0]: 1
list[1]: 2
list[2]: 3

dictionary[1]: one
dictionary[2]: two
dictionary[3]: three

tuple[0]: 1
tuple[1]: 2
tuple[2]: 3

'set' object does not support indexing

Tuple Assignment

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Python supports having a tuple on the left side of an assignment statement. This allows you to assign more than one variable at a time.[11]

x = 1
y = 2
print(x, y)

x, y = y, x
print(x, y)

Output:

1 2
2 1

Set Operations

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Sets support logical set operations, including membership, subset, superset, union (|), intersection (&), and difference (-).[12]

numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
even = {2, 4, 6, 8, 10}
odd = {1, 3, 5, 7, 9}

print("Numbers:", numbers)
print("Even:", even)
print("Odd:", odd)

print("1 in numbers:", 1 in numbers)
print("even.issubset(numbers):", even.issubset(numbers))
print("numbers.issuperset(odd):", numbers.issuperset(odd))
print("even | odd:", even | odd)
print("even & odd:", even & odd)
print("numbers - even:", numbers - even)

Output:

Numbers: {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
Even: {8, 10, 2, 4, 6}
Odd: {9, 1, 3, 5, 7}
1 in numbers: True
even.issubset(numbers): True
numbers.issuperset(odd): True
even | odd: {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
even & odd: set()
numbers - even: {1, 9, 3, 5, 7}

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Practice

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  1. Create a Python program that contains a dictionary of names and phone numbers. Use a tuple of separate first and last name values for the key field. Initialize the dictionary with at least three names and numbers. Ask the user to search for a phone number by entering a first and last name. Display the matching number if found, or a message if not found.
  2. Create a Python program that contains a dictionary of names and phone numbers. Use a tuple of separate first and last name values for the key field. Allow the user to enter names and numbers and add them to the dictionary. When the user is finished adding entries, display the names and numbers sorted in alphabetical order by last name and first name.
  3. Create a Python program that asks the user to enter a line of text. Use the string split() method to parse the line and then display the list of words in alphabetical order with any duplicate words removed from the list. Use set() to remove the duplicates.
  4. Create a Python program that asks the user to enter two sets of comma-separated values. Use the string split() method to parse the line and then use the set() function to covert the lists to sets. Demonstrate set theory for the two sets by displaying the two sets and their relationship to each other as subset, superset, union, intersection, and difference.

Lesson Summary

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Data Structure Concepts

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  • A data structure is a particular way of organizing data in a computer so that it can be used efficiently.[13]
  • Data structures can be used to organize the storage and retrieval of information stored in both main memory and secondary memory.[14]
  • The array and record data structures are based on computing the addresses of data items with arithmetic operations; while the linked data structures are based on storing addresses of data items within the structure itself.[15]
  • An array is a number of elements in a specific order, typically all of the same type. Elements are accessed using an integer index to specify which element is required. Arrays may be fixed-length or resizable.[16]
  • A linked list is a linear collection of data elements of any type, called nodes, where each node has itself a value, and points to the next node in the linked list.[17]
  • A record (also called tuple or struct) is an aggregate data structure. A record is a value that contains other values, typically in fixed number and sequence and typically indexed by names. The elements of records are usually called fields or members.[18]
  • A class is a data structure that contains data fields, like a record, as well as various methods which operate on the contents of the record.[19]

Python Data Structures

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  • Python data structures include lists, dictionaries, tuples, and sets.[20]
  • Python lists are implemented internally as variable-length arrays, rather than linked lists.[21]
  • Python dictionaries are implemented internally as resizable hash tables.[22]
  • Python tuples are records.[23]
  • Python sets are implemented internally as hash tables (like dictionaries) with optimizations that take advantage of the fact that the values are always None.[24]
  • The data types for a list, dictionary, tuple, and set, are 'list', 'dict', 'tuple', and 'set', respectively.[25]
  • The list(), dict(), tuple(), and set() functions may be used to create list, dictionary, tuple, and set objects, respectively.[26]
  • List and dictionary items are mutable. Tuple and set items are immutable.[27]
  • Lists and tuples maintain order. Dictionaries and sets are unordered.[28]
  • Lists and tuples allow duplication. Dictionary and set items are unique.[29]
  • Sets may be used to remove duplication found in other data structures.[30]
  • List, dictionary, tuple, and set items may be accessed using a for loop.[31]
  • List and tuple items may be accessed by index. Dictionary items are accessed by key. Set items cannot be accessed by index.[32]
  • Python supports having a tuple on the left side of an assignment statement. This allows you to assign more than one variable at a time.[33]
  • Sets support logical set operations, including membership, subset, superset, union, intersection, and difference.[34]

Key Terms

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comparable
A type where one value can be checked to see if it is greater than, less than, or equal to another value of the same type. Types which are comparable can be put in a list and sorted.[35]
data structure
A collection of related values, often organized in lists, dictionaries, tuples, etc.[36]
DSU
Abbreviation of “decorate-sort-undecorate”, a pattern that involves building a list of tuples, sorting, and extracting part of the result.[37]
gather
The operation of assembling a variable-length argument tuple.[38]
hashable
A type that has a hash function. Immutable types like integers, floats, and strings are hashable; mutable types like lists and dictionaries are not.[39]
scatter
The operation of treating a sequence as a list of arguments.[40]
shape (of a data structure)
A summary of the type, size, and composition of a data structure.[41]
singleton
A list (or other sequence) with a single element.[42]
tuple
An immutable sequence of elements.[43]
tuple assignment
An assignment with a sequence on the right side and a tuple of variables on the left. The right side is evaluated and then its elements are assigned to the variables on the left.[44]

Review Questions

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Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. A data structure is _____.
    A data structure is a particular way of organizing data in a computer so that it can be used efficiently.
  2. Data structures can be used to _____.
    Data structures can be used to organize the storage and retrieval of information stored in both main memory and secondary memory.
  3. The array and record data structures are based on _____; while the linked data structures are based on _____.
    The array and record data structures are based on computing the addresses of data items with arithmetic operations; while the linked data structures are based on storing addresses of data items within the structure itself.
  4. An array is _____. Elements are accessed using _____. Arrays may be _____.
    An array is a number of elements in a specific order, typically all of the same type. Elements are accessed using an integer index to specify which element is required. Arrays may be fixed-length or resizable.
  5. A linked list is _____.
    A linked list is a linear collection of data elements of any type, called nodes, where each node has itself a value, and points to the next node in the linked list.
  6. A record (also called tuple or struct) is _____. The elements of records are usually called _____.
    A record (also called tuple or struct) is an aggregate data structure. A record is a value that contains other values, typically in fixed number and sequence and typically indexed by names. The elements of records are usually called fields or members.
  7. A class is _____.
    A class is a data structure that contains data fields, like a record, as well as various methods which operate on the contents of the record.
  8. Python data structures include _____.
    Python data structures include lists, dictionaries, tuples, and sets.
  9. Python lists are implemented internally as _____.
    Python lists are implemented internally as variable-length arrays, rather than linked lists.
  10. Python dictionaries are implemented internally as _____.
    Python dictionaries are implemented internally as resizable hash tables.
  11. Python tuples are _____.
    Python tuples are records.
  12. Python sets are implemented internally as _____.
    Python sets are implemented internally as dictionaries (hash tables) with keys and no values.
  13. The data types for a list, dictionary, tuple, and set, are _____.
    The data types for a list, dictionary, tuple, and set, are 'list', 'dict', 'tuple', and 'set', respectively.
  14. The list(), dict(), tuple(), and set() functions may be used to _____.
    The list(), dict(), tuple(), and set() functions may be used to create list, dictionary, tuple, and set objects, respectively.
  15. List and dictionary items are _____. Tuple and set items are _____.
    List and dictionary items are mutable. Tuple and set items are immutable.
  16. Lists and tuples maintain _____. Dictionaries and sets are _____.
    Lists and tuples maintain order. Dictionaries and sets are unordered.
  17. Lists and tuples allow _____. Dictionary and set items _____.
    Lists and tuples allow duplication. Dictionary and set items are unique.
  18. Sets may be used to remove _____.
    Sets may be used to remove duplication found in other data structures.
  19. List, dictionary, tuple, and set items may be accessed using a _____ loop.
    List, dictionary, tuple, and set items may be accessed using a for loop.
  20. List and tuple items may be accessed by _____. Dictionary items are accessed by _____. Set items _____.
    List and tuple items may be accessed by index. Dictionary items are accessed by key. Set items cannot be accessed by index.
  21. Python supports having a tuple on the left side of an assignment statement. This allows you to _____.
    Python supports having a tuple on the left side of an assignment statement. This allows you to assign more than one variable at a time.
  22. Sets support logical set operations, including _____.
    Sets support logical set operations, including membership, subset, superset, union, intersection, and difference.

Assessments

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See Also

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References

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Lesson 10 - Classes

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This lesson introduces Python classes.


Objectives and Skills

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Objectives and skills for this lesson include:[1]

Readings

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  1. Wikipedia: Object-oriented programming
  2. Sthurlow: Python Classes
  3. Jess Hamrick: An Introduction to Classes and Inheritance (in Python)

Multimedia

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  1. YouTube: Python Classes and Objects
  2. YouTube: Python init
  3. YouTube: Python Class vs. Instance Variables
  4. YouTube: Python Inheritance
  5. YouTube: Python Classes and Objects Playlist

Examples

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Class Definition

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Classes are defined using the syntax:[2]

class ClassName:
    <statements>

Class Instantiation

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Classes are instantiated using the syntax:[3]

variable = ClassName()

Class Attributes

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Class attributes (variables) are defined using the syntax:[4]

class ClassName:
    attribute = value

Class Methods

edit

Class methods (functions) are defined using the syntax:[5]

class ClassName:
    def function(self):
        <statements>

Class Initialization

edit

Class attributes may be initialized during instantiation using the __init__() method.[6]

class ClassName:
    def __init__(self, attribute[, attribute, ...]):
        <statements>

Class Example

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The following example creates a class named Shape with an attribute named sides and a method named perimeter. Three instances of the class are created, one each for a triangle, rectangle, and square. The attribute value and method result are printed for each shape.

class Shape(object):
    sides = None

    def __init__(self, sides):
        self.sides = sides

    def perimeter(self):
        perimeter = 0
        for side in self.sides:
            perimeter += side
        return perimeter

triangle = Shape([3, 4, 5])
print("triangle sides:", triangle.sides)
print("triangle perimeter:", triangle.perimeter())

rectangle = Shape([4, 2, 4, 2])
print("rectangle sides:", rectangle.sides)
print("rectangle perimeter:", rectangle.perimeter())

square = Shape([2, 2, 2, 2])
print("square sides:", square.sides)
print("square perimeter:", square.perimeter())

Output:

triangle sides: [3, 4, 5]
triangle perimeter: 12
rectangle sides: [4, 2, 4, 2]
rectangle perimeter: 12
square sides: [2, 2, 2, 2]
square perimeter: 8

Inheritance

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Subclasses are defined using the syntax:[7]

class DerivedClassName(BaseClassName):
    <statements>

Inheritance Example

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The following example creates a class named Shape with an attribute named sides and a method named perimeter. Three subclasses are created named Triangle, Rectangle, and Square. An instance of each subclass is created, and the attribute value and method result are printed for each shape.

class Shape(object):
    sides = None

    def __init__(self, sides):
        self.sides = sides

    def perimeter(self):
        perimeter = 0
        for side in self.sides:
            perimeter += side
        return perimeter

class Triangle(Shape):
    def __init__(self, side1, side2, side3):
        self.sides = [side1, side2, side3]

class Rectangle(Shape):
    def __init__(self, length, width):
        self.sides = [length, width, length, width]

class Square(Shape):
    def __init__(self, side):
        self.sides = [side, side, side, side]

triangle = Triangle(3, 4, 5)
print("triangle sides:", triangle.sides)
print("triangle perimeter:", triangle.perimeter())

rectangle = Rectangle(4, 2)
print("rectangle sides:", rectangle.sides)
print("rectangle perimeter:", rectangle.perimeter())

square = Square(2)
print("square sides:", square.sides)
print("square perimeter:", square.perimeter())

Output:

triangle sides: [3, 4, 5]
triangle perimeter: 12
rectangle sides: [4, 2, 4, 2]
rectangle perimeter: 12
square sides: [2, 2, 2, 2]
square perimeter: 8

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Practice

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  1. Create a Python program that asks the user how old they are in years. Then ask the user if they would like to know how old they are in months, days, hours, or seconds. Use a condition statement to determine their selection, and display their approximate age in the selected timeframe. Perform all calculations using an AgeConverter class that accepts the age in years during initialization and has separate methods that calculate and return the age in months, days, hours, and seconds. Include data validation in the class and error handling in the main program.
  2. Review MathsIsFun: Conversion of Temperature. Create a Python program that asks the user if they would like to convert Fahrenheit to Celsius or Celsius to Fahrenheit. Use a condition statement to determine their selection and then gather the appropriate input. Perform all calculations using a TemperatureConverter class that accepts the temperature during initialization and has separate methods that calculate and return the temperature in Celsius and Fahrenheit. Include data validation in the class and error handling in the main program.
  3. Review MathsIsFun: Area of Plane Shapes. Create a Python program that asks the user what shape they would like to calculate the area for. Use a condition statement to determine their selection and then gather the appropriate input. Perform all area calculations using a Shape main class and subclasses for each selected shape type. Include data validation in the class and error handling in the main program.
  4. Create a Python program that contains a dictionary of names and phone numbers. Use a class to contain and maintain the dictionary, and use methods to add, update, remove, and search for names and numbers. Include data validation in the class and error handling in the main program.

Lesson Summary

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Class Concepts

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  • Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which may contain data, in the form of fields, often known as attributes; and code, in the form of procedures, often known as methods.[8]
  • A feature of objects is that an object's procedures can access and often modify the data fields of the object with which they are associated through a notion of "this" or "self".[9]
  • Classes define the data format and available procedures for a given type of object and may also contain data and procedures (known as class attributes and methods).[10]
  • Objects are instances of classes.[11]
  • Class variables belong to the class as a whole; there is only one copy of each one.[12]
  • Instance variables or attributes belong to individual objects; every object has its own copy of each one.[13]
  • Class methods belong to the class as a whole and have access only to class variables and inputs from the procedure call.[14]
  • Instance methods belong to individual objects, and have access to instance variables for the specific object they are called on, inputs, and class variables.[15]
  • Classes have a unique namespace so code in one class will not be accidentally confused with the same procedure or variable name in another class.[16]
  • Classes provide a layer of abstraction which can be used to separate internal from external code. External code can use an object by calling a specific instance method with a certain set of input parameters, read an instance variable, or write to an instance variable.[17]
  • Objects are created by calling a special type of method in the class known as a constructor.[18]
  • A program may create many instances of the same class as it runs, which operate independently. This is an easy way for the same procedures to be used on different sets of data.[19]
  • Inheritance allows classes to be arranged in a hierarchy that represents "is-a-type-of" relationships. All data and methods available to the parent class also appear in the child class with the same names.[20]
  • Subclasses can override the methods defined by superclasses.[21]

Python Classes

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  • Classes are defined using the syntax:[22]

    class ClassName:
        <statements>

  • Classes are instantiated using the syntax:[23]

    variable = ClassName()

  • Class attributes (variables) are defined using the syntax:[24]

    class ClassName:
        attribute = value

  • Class methods (functions) are defined using the syntax:[25]
        class ClassName:
            def function(self):
                <statements>
  • Class attributes may be initialized during instantiation using the __init__() method.[26]
        class ClassName:
            def __init__(self, attribute[, attribute, ...]):
                <statements>
  • Subclasses are defined using the syntax:[27]
        class DerivedClassName(BaseClassName):
            <statements>

Key Terms

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attribute
Data encapsulated within a class or object.[28]
class
An extensible program-code-template for creating objects, providing initial values for state (member variables) and implementations of behavior (member functions or methods).[29]
constructor
A special type of procedure called to create an object which prepares the new object for use, often accepting arguments that the constructor uses to set required member variables.[30]
inheritance
When an object or class is based on another object or class using the same implementation to maintain the same behavior.[31]
instance
A concrete occurrence of an object.[32]
instantiate
To create an occurrence of an object, typically by calling its constructor.[33]
method
A procedure associated with an object.[34]
object
A particular instance of a class.[35]
subclass
A modular, derivative class that inherits one or more language entities from one or more other classes (called superclasses, base classes, or parent classes).[36]

Review Questions

edit
Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. Object-oriented programming (OOP) is _____.
    Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which may contain data, in the form of fields, often known as attributes; and code, in the form of procedures, often known as methods.
  2. A feature of objects is _____.
    A feature of objects is that an object's procedures can access and often modify the data fields of the object with which they are associated through a notion of "this" or "self".
  3. Classes define _____.
    Classes define the data format and available procedures for a given type of object and may also contain data and procedures (known as class attributes and methods).
  4. Objects are _____.
    Objects are instances of classes.
  5. Class variables belong to _____.
    Class variables belong to the class as a whole; there is only one copy of each one.
  6. Instance variables or attributes belong to _____.
    Instance variables or attributes belong to individual objects; every object has its own copy of each one.
  7. Class methods belong to _____.
    Class methods belong to the class as a whole and have access only to class variables and inputs from the procedure call.
  8. Instance methods belong to _____.
    Instance methods belong to individual objects, and have access to instance variables for the specific object they are called on, inputs, and class variables.
  9. Classes have a unique _____.
    Classes have a unique namespace so code in one class will not be accidentally confused with the same procedure or variable name in another class.
  10. Classes provide a layer of abstraction which _____.
    Classes provide a layer of abstraction which can be used to separate internal from external code. External code can use an object by calling a specific instance method with a certain set of input parameters, read an instance variable, or write to an instance variable.
  11. Objects are created by calling _____.
    Objects are created by calling a special type of method in the class known as a constructor.
  12. A program may create many _____ of the same class as it runs, which operate independently. This is an easy way for the same procedures to be used on different sets of data.
    A program may create many instances of the same class as it runs, which operate independently. This is an easy way for the same procedures to be used on different sets of data.
  13. Inheritance allows classes to _____.
    Inheritance allows classes to be arranged in a hierarchy that represents "is-a-type-of" relationships. All data and methods available to the parent class also appear in the child class with the same names.
  14. Subclasses can override _____.
    Subclasses can override the methods defined by superclasses.
  15. Classes are defined using the syntax:
    Classes are defined using the syntax:
        class ClassName:
            <statements>
  16. Classes are instantiated using the syntax:
    {{{2}}}
  17. Class attributes (variables) are defined using the syntax:
    {{{2}}}
  18. Class methods (functions) are defined using the syntax:
    Class methods (functions) are defined using the syntax:
        class ClassName:
            def function(self):
                <statements>
  19. Class attributes may be initialized during instantiation using the ____ method.
    Class attributes may be initialized during instantiation using the __init__() method.
        class ClassName:
            def __init__(self, attribute[, attribute, ...]):
                <statements>
  20. Subclasses are defined using the syntax:
    Subclasses are defined using the syntax:
        class DerivedClassName(BaseClassName):
            <statements>

Assessments

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See Also

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References

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Lesson 11 - Modules

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This lesson introduces Python modules and packages.


Objectives and Skills

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Objectives and skills for this lesson include:[1]

  • Modules
    • Byte-compiled .pyc files
    • The from ..import ..statement
    • A module's __name__ and custom modules
    • The dir function and packages

Readings

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  1. Wikipedia: Modular programming
  2. Python.org: Modules

Multimedia

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  1. YouTube: Python Modules

Examples

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Modules

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Modules are regular Python .py files that define functions or classes that may be imported and reused in other Python programs.[2]

functions.py:

"""This module contains functions that will be imported and reused."""

def function1():
    """Function1"""
    print("In function1")

classes.py:

"""This module contains classes that will be imported and reused."""

class Class1(object):
    """Class1"""

    def method1(self):
        """method1"""
        print("In method1")

The import Statement

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The import statement finds a module, loads it, initializes it if necessary, and defines a name or names which are used to reference code in the module.[3]

import functions
import classes

functions.function1()

class1 = classes.Class1()
class1.method1()

Output:

In function1
In method1

The from ... import Statement

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The from...import statement finds a module, loads it, initializes it if necessary, and then adds module references to the local namespace, allowing functions and classes to be accessed without a module reference.[4] Use of import rather than from...import is preferred. While from ... import supports an * option to import all references rather than naming specific functions or classes, this use is discouraged.[5]

from functions import function1
from classes import Class1

function1()

class1 = Class1()
class1.method1()

Output:

In function1
In method1

Module Name

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When modules are imported, the __name__ variable is set to the name of the module. When the Python interpreter runs a module directly, the __name__ variable is set to "__main__". This allows a module designed to be imported to add a main() function that will only execute when the module is run directly.[6][7]

def main():
    """Used to demonstrate and/or test module code."""
    ...

if __name__ == "__main__":
    main()

The dir() Function

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The dir() function returns the list of names in the current local scope, or for the object, if specified.[8]

print(dir())

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Practice

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  1. Create a module with at least one function or class. In a separate module, import the first module and use the dir() function to display the names in the local scope. Then import the first module using from ... import * and use the dir() function to display the names in the local scope. Compare the difference between import and from ... import.
  2. Modify one or more of the Python Programming/Functions#Practice activities so that the function code is in a different module. Use import to import the module and then call the function.
  3. Modify one or more of the Python Programming/Classes#Practice activities so that the class code is in a different module. Use import to import the module and then instantiate and use the class.
  4. Review Python.org: os. Import the Python os library and then use getlogin() and uname() to display the current username, computer name, operating system, and version.

Lesson Summary

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  • Modular programming is a software design technique that emphasizes separating the functionality of a program into independent, interchangeable modules, such that each contains everything necessary to execute only one aspect of the desired functionality.[9]
  • Modules are regular Python .py files that define functions or classes that may be imported and reused in other Python programs.[10]
  • The import statement finds a module, loads it, initializes it if necessary, and defines a name or names which are used to reference code in the module.[11]
  • The from...import statement finds a module, loads it, initializes it if necessary, and then adds module references to the local namespace, allowing functions and classes to be accessed without a module reference.[12]
  • Use of import rather than from...import is preferred.[13]
  • While from ... import supports an * option to import all references rather than naming specific functions or classes, this use is discouraged.[14]
  • When modules are imported, the __name__ variable is set to the name of the module. When the Python interpreter runs a module directly, the __name__ variable is set to "__main__".[15][16]
  • __name__ allows a module designed to be imported to add a main() function that will only execute when the module is run directly.[17][18]
  • To speed up loading modules, Python caches the compiled bytecode version of each module.[19]
  • A program doesn’t run any faster when it is read from a .pyc file than when it is read from a .py file; the only thing that’s faster about .pyc files is the speed with which they are loaded.[20]
  • Python comes with a library of standard modules.[21]
  • The sys module provides access to operating system or operating environment variables and functions.[22]
  • The dir() function returns the list of names in the current local scope, or for the object, if specified.[23]

Key Terms

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bytecode
A form of instruction set designed for efficient execution by a software interpreter.[24]

Review Questions

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Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. Modular programming is _____.
    Modular programming is a software design technique that emphasizes separating the functionality of a program into independent, interchangeable modules, such that each contains everything necessary to execute only one aspect of the desired functionality.
  2. Python modules are _____.
    Python modules are regular Python .py files that define functions or classes that may be imported and reused in other Python programs.
  3. The import statement _____.
    The import statement finds a module, loads it, initializes it if necessary, and defines a name or names which are used to reference code in the module.
  4. The from...import statement _____.
    The from...import statement finds a module, loads it, initializes it if necessary, and then adds module references to the local namespace, allowing functions and classes to be accessed without a module reference.
  5. When importing Python modules, use of _____ rather than _____ is preferred.
    When importing Python modules, se of import rather than from...import is preferred.
  6. While from ... import supports _____, this use is discouraged.
    While from ... import supports an * option to import all references rather than naming specific functions or classes, this use is discouraged.
  7. When modules are imported, the __name__ variable is set to _____. When the Python interpreter runs a module directly, the __name__ variable is set to _____.
    When modules are imported, the __name__ variable is set to the name of the module. When the Python interpreter runs a module directly, the __name__ variable is set to "__main__".
  8. __name__ allows a module designed to be imported to add _____.
    __name__ allows a module designed to be imported to add a main() function that will only execute when the module is run directly.
  9. To speed up loading modules, Python _____.
    To speed up loading modules, Python caches the compiled bytecode version of each module.
  10. A program doesn’t run any faster when it is read from a _____ file than when it is read from a _____ file; the only thing that’s faster about _____ files is _____.
    A program doesn’t run any faster when it is read from a .pyc file than when it is read from a .py file; the only thing that’s faster about .pyc files is the speed with which they are loaded.
  11. Python comes with _____.
    Python comes with a library of standard modules.
  12. The sys module _____.
    The sys module provides access to operating system or operating environment variables and functions.
  13. The dir() function _____.
    The dir() function returns the list of names in the current local scope, or for the object, if specified.

Assessments

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See Also

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References

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Lesson 12 - RegEx

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This lesson introduces Python regular expression processing.


Objectives and Skills

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Objectives and skills for this lesson include:

  • Standard Library
    • Regular expression operations

Readings

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  1. Wikipedia: Regular expression
  2. Python for Everyone: Regular expressions

Multimedia

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  1. YouTube: Python for Informatics - Chapter 11 - Regular Expressions
  2. YouTube: Python - Regular Expressions
  3. YouTube: Python3 - Regular Expressions

Examples

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The match() Method

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The match() method looks for zero or more characters at the beginning of the given string that match the given regular expression and returns a match object if found, or None if there is no match.[1]

import re

string = "<p>HTML text.</p>"
match = re.match("<p>.*</p>", string)
if match:
    print("start:", match.start(0))
    print("end:", match.end(0))
    print("group:", match.group(0))

Output:

start: 0
end: 17
group: <p>HTML text.</p>

The search() Method

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The search() method scans for the first match of the given regular expression in the given string and returns a match object if found, or None if there is no match.[2]

import re

string = "<h1>Heading</h1><p>HTML text.</p>"
match = re.search("<p>.*</p>", string)
if match:
    print("start:", match.start(0))
    print("end:", match.end(0))
    print("group:", match.group(0))

Output:

start: 16
end: 33
group: <p>HTML text.</p>

Greedy vs. Non-greedy

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The '*', '+', and '?' quantifiers are all greedy; they match as much text as possible. Adding ? after the quantifier makes it perform the match in non-greedy or minimal fashion; as few characters as possible will be matched.[3]

import re

string = "<h1>Heading</h1><p>HTML text.</p>"

match = re.search("<.*>", string)
if match:
    print("Greedy")
    print("start:", match.start(0))
    print("end:", match.end(0))
    print("group:", match.group(0))

match = re.search("<.*?>", string)
if match:
    print("\nNon-greedy")
    print("start:", match.start(0))
    print("end:", match.end(0))
    print("group:", match.group(0))

Output:

Greedy
start: 0
end: 33
group: <h1>Heading</h1><p>HTML text.</p>

Non-greedy
start: 0
end: 4
group: <h1>

The findall() Method

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The findall() method matches all occurrences of the given regular expression in the string and returns a list of matching strings.[4]

import re

string = "<h1>Heading</h1><p>HTML text.</p>"
matches = re.findall("<.*?>", string)
print("matches:", matches)

Output:

matches: ['<h1>', '</h1>', '<p>', '</p>']

The sub() Method

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The sub() method replaces every occurrence of a pattern with a string.[5]

import re

string = "<h1>Heading</h1><p>HTML text.</p>"
string = re.sub("<.*?>", "", string)
print("string:", string)

Output:

string: HeadingHTML text.

The split() Method

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The split() method splits string by the occurrences of pattern.[6]

import re

string = "cat: Frisky, dog: Spot, fish: Bubbles"
keys = re.split(": ?\w*,? ?", string)
values = re.split(",? ?\w*: ?", string)
print("string:", string)
print("keys:", keys)
print("values:", values)

Output:

string: cat: Frisky, dog: Spot, fish: Bubbles
keys: ['cat', 'dog', 'fish', '']
values: ['', 'Frisky', 'Spot', 'Bubbles']

The compile() Method

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The compile() method compiles a regular expression pattern into a regular expression object, which can be used for matching using its match() and search() methods. The expression’s behaviour can be modified by specifying a flags value.[7]

import re

string = "<p>Lines of<br>HTML text</p>"
regex = re.compile("<br>", re.IGNORECASE)
match = regex.search(string)
if match:
    print("start:", match.start(0))
    print("end:", match.end(0))
    print("group:", match.group(0))

Output:

start: 11
end: 15
group: <br>

Match Groups

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Match groups match whatever regular expression is inside parentheses, and indicates the start and end of a group; the contents of a group can be retrieved after a match has been performed.[8]

import re

string = "<p>HTML text.</p>"
match = re.match("<p>(.*)</p>", string)
if match:
    print("start:", match.start(1))
    print("end:", match.end(1))
    print("group:", match.group(1))

string = "'cat': 'Frisky', 'dog': 'Spot', 'fish': 'Bubbles'"

match = re.search("'cat': '(.*?)', 'dog': '(.*?)', 'fish': '(.*?)'", string)
if match:
    print("groups:", match.group(1), match.group(2), match.group(3))
    
lst = re.findall(r"'(.*?)': '(.*?)',?\s*", string)
for key, value in lst:
  print("%s: %s" % (key, value))

Output:

start: 3
end: 13
group: HTML text.
groups: Frisky Spot Bubbles
cat: Frisky
dog: Spot
fish: Bubbles

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Practice

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  1. Create a Python program that asks the user to enter a line of comma-separated grade scores. Use RegEx methods to parse the line and add each item to a list. Display the list of entered scores sorted in descending order and then calculate and display the high, low, and average for the entered scores. Include try and except to handle input errors.
  2. Create a Python program that asks the user for a line of text that contains HTML tags, such as:
        <p><strong>This is a bold paragraph.</strong></p>
    Use RegEx methods to search for and remove all HTML tags from the text, saving each removed tag in a list. Print the untagged text and then display the list of removed tags sorted in alphabetical order with duplicate tags removed. Include error handling in case an HTML tag isn't entered correctly (an unmatched < or >). Use a user-defined function for the actual string processing, separate from input and output.
  3. Create a Python program that asks the user to enter a line of dictionary keys and values in the form Key-1: Value 1, Key-2: Value 2, Key-3: Value 3. You may assume that keys will never contain spaces, but may contain hyphens. Values may contain spaces, but a comma will always separate one key-value pair from the next key-value pair. Use RegEx functions to parse the string and build a dictionary of key-value pairs. Then display the dictionary sorted in alphabetical order by key. Include input validation and error handling in case a user accidentally enters the same key more than once.

Lesson Summary

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RegEx Concepts

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  • A regular expression (abbreviated regex) is a sequence of characters that forms a search pattern, mainly for use in pattern matching with strings.[9]
  • Each character in a regular expression is either understood to be a metacharacter with its special meaning, or a regular character with its literal meaning.[10]
  • In regex, | indicates either|or.[11]
  • In regex, ? indicates there is zero or one of the preceding element.[12]
  • In regex, * indicates there is zero or more of the preceding element.[13]
  • In regex, + indicates there is one or more of the preceding element.[14]
  • In regex, () is used to group elements.[15]
  • In regex, . matches any single character.[16]
  • In regex, [] matches any single character contained within the brackets.[17]
  • In regex, [^] matches any single character not contained within the brackets.[18]
  • In regex, ^ matches the start of the string.[19]
  • In regex, $ matches the end of the string.[20]
  • In regex, \w matches a word.[21]
  • In regex, \d matches a digit.[22]
  • In regex, \s matches whitespace.[23]

Python RegEx

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  • The Python regular expression library is re.py, and accessed using import re.[24]
  • The match() method looks for zero or more characters at the beginning of the given string that match the given regular expression and returns a match object if found, or None if there is no match.[25]
  • The search() method scans for the first match of the given regular expression in the given string and returns a match object if found, or None if there is no match.[26]
  • The '*', '+', and '?' quantifiers are all greedy; they match as much text as possible. Adding ? after the quantifier makes it perform the match in non-greedy or minimal fashion; as few characters as possible will be matched.[27]
  • The findall() method matches all occurrences of the given regular expression in the string and returns a list of matching strings.[28]
  • The sub() method replaces every occurrence of a pattern with a string.[29]
  • The split() method splits string by the occurrences of pattern.[30]
  • The compile() method compiles a regular expression pattern into a regular expression object, which can be used for matching using its match() and search() methods. The expression’s behaviour can be modified by specifying a flags value.[31]
  • The compile() method flags include re.IGNORECASE, re.MULTILINE, and re.DOTALL for case insensitivity and processing more than one line at a time.[32]
  • Match groups match whatever regular expression is inside parentheses, and indicates the start and end of a group; the contents of a group can be retrieved after a match has been performed.[33]

Key Terms

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brittle code
Code that works when the input data is in a particular format but is prone to breakage if there is some deviation from the correct format. We call this “brittle code” because it is easily broken.[34]
greedy matching
The notion that the “+” and “*” characters in a regular expression expand outward to match the largest possible string.[35]
grep
A command available in most Unix systems that searches through text files looking for lines that match regular expressions. The command name stands for "Generalized Regular Expression Parser".[36]
regular expression
A language for expressing more complex search strings. A regular expression may contain special characters that indicate that a search only matches at the beginning or end of a line or many other similar capabilities.[37]
wild card
A special character that matches any character. In regular expressions the wild-card character is the period.[38]

Review Questions

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Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. A regular expression (abbreviated regex) is _____.
    A regular expression (abbreviated regex) is a sequence of characters that forms a search pattern, mainly for use in pattern matching with strings.
  2. Each character in a regular expression is either _____, or _____.
    Each character in a regular expression is either understood to be a metacharacter with its special meaning, or a regular character with its literal meaning.
  3. In regex, | indicates _____.
    In regex,
  4. In regex, ? indicates _____.
    In regex, ? indicates there is zero or one of the preceding element.
  5. In regex, * indicates _____.
    In regex, * indicates there is zero or more of the preceding element.
  6. In regex, + indicates _____.
    In regex, + indicates there is one or more of the preceding element.
  7. In regex, () is used to _____.
    In regex, () is used to group elements.
  8. In regex, . matches _____.
    In regex, . matches any single character.
  9. In regex, [] matches _____.
    In regex, [] matches any single character contained within the brackets.
  10. In regex, [^] matches _____.
    In regex, [^] matches any single character not contained within the brackets.
  11. In regex, ^ matches _____.
    In regex, ^ matches the start of the string.
  12. In regex, $ matches _____.
    In regex, $ matches the end of the string.
  13. In regex, \w matches _____.
    In regex, \w matches a word.
  14. In regex, \d matches _____.
    In regex, \d matches a digit.
  15. In regex, \s matches _____.
    In regex, \s matches whitespace.
  16. The match() method _____.
    The match() method looks for zero or more characters at the beginning of the given string that match the given regular expression and returns a match object if found, or None if there is no match.
  17. The search() method _____.
    The search() method scans for the first match of the given regular expression in the given string and returns a match object if found, or None if there is no match.
  18. The '*', '+', and '?' quantifiers are all _____; they match _____. Adding ? after the quantifier makes it _____.
    The '*', '+', and '?' quantifiers are all greedy; they match as much text as possible. Adding ? after the quantifier makes it perform the match in non-greedy or minimal fashion; as few characters as possible will be matched.
  19. The findall() method _____.
    The findall() method matches all occurrences of the given regular expression in the string and returns a list of matching strings.
  20. The sub() method _____.
    The sub() method replaces every occurrence of a pattern with a string.
  21. The split() method _____.
    The split() method splits string by the occurrences of pattern.
  22. The compile() method _____.
    The compile() method compiles a regular expression pattern into a regular expression object, which can be used for matching using its match() and search() methods. The expression’s behaviour can be modified by specifying a flags value.
  23. The compile() method flags include _____.
    The compile() method flags include re.IGNORECASE, re.MULTILINE, and re.DOTALL for case insensitivity and processing more than one line at a time.
  24. Match groups match _____.
    Match groups match whatever regular expression is inside parentheses, and indicates the start and end of a group; the contents of a group can be retrieved after a match has been performed.

Assessments

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See Also

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References

edit
  1. Python.org: Regular expression operations
  2. Python.org: Regular expression operations
  3. Python.org: Regular expression operations
  4. Python.org: Regular expression operations
  5. Python.org: Regular expression operations
  6. Python.org: Regular expression operations
  7. Python.org: Regular expression operations
  8. Python.org: Regular expression operations
  9. Wikipedia: Regular expression
  10. Wikipedia: Regular expression
  11. Wikipedia: Regular expression
  12. Wikipedia: Regular expression
  13. Wikipedia: Regular expression
  14. Wikipedia: Regular expression
  15. Wikipedia: Regular expression
  16. Wikipedia: Regular expression
  17. Wikipedia: Regular expression
  18. Wikipedia: Regular expression
  19. Wikipedia: Regular expression
  20. Wikipedia: Regular expression
  21. Wikipedia: Regular expression
  22. Wikipedia: Regular expression
  23. Wikipedia: Regular expression
  24. Python.org: Regular expression operations
  25. Python.org: Regular expression operations
  26. Python.org: Regular expression operations
  27. Python.org: Regular expression operations
  28. Python.org: Regular expression operations
  29. Python.org: Regular expression operations
  30. Python.org: Regular expression operations
  31. Python.org: Regular expression operations
  32. Python.org: Regular expression operations
  33. Python.org: Regular expression operations
  34. PythonLearn: Regular expressions
  35. PythonLearn: Regular expressions
  36. PythonLearn: Regular expressions
  37. PythonLearn: Regular expressions
  38. PythonLearn: Regular expressions

Lesson 13 - Files

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This lesson introduces Python file processing.


Objectives and Skills

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Objectives and skills for this lesson include:[1]

  • Standard Library
    • os module
    • sys module
  • Input Output
    • Files I/O

Readings

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  1. Wikipedia: File system
  2. Wikipedia: Directory (computing)
  3. Wikipedia: Directory structure
  4. Wikipedia: Text file
  5. PythonLearn: Automating common tasks on your computer
  6. Python for Everyone: Files

Multimedia

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  1. YouTube: Python for Informatics - Chapter 7 Files
  2. YouTube: Python - How to Read and Write Files

Examples

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The os.getcwd() Method

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The os.getcwd() method returns a string representing the current working directory.[2]

import os

print("Current working directory:", os.getcwd())

Output:

Current working directory: /home/ubuntu/workspace

The os.chdir() Method

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The os.chdir() method changes the current working directory to the given path.[3]

import os

directory = os.getcwd()
print("Current working directory:", directory)
os.chdir("..")
print("Changed to:", os.getcwd())
os.chdir(directory)
print("Changed back to:", directory)

Output:

Current working directory: /home/ubuntu/workspace
Changed to: /home/ubuntu
Changed back to: /home/ubuntu/workspace

The os.path.isdir() Method

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The os.path.isdir() method returns True if the given path is an existing directory.[4]

import os

path = os.getcwd()
if os.path.isdir(path):
    print("Current working directory exists.")
else:
    print("Current working directory does not exist.")

Output:

Current working directory exists.

The os.path.join() Method

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The os.path.join() method joins one or more path components intelligently, avoiding extra directory separator (os.sep) characters.[5]

import os

path = os.getcwd()
directory = os.path.join(path, "__python_demo__")
print("path:", path)
print("directory:", directory)

Output:

path: /home/ubuntu/workspace
directory: /home/ubuntu/workspace/__python_demo__

The os.mkdir() Method

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The os.mkdir() method creates a directory with the given path.[6]

import os

path = os.getcwd()

directory = os.path.join(path, "__python_demo__")
if os.path.isdir(directory):
    raise Exception("Path already exists. Can't continue.")
os.mkdir(directory)

The os.rmdir() Method

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The os.rmdir() method removes (deletes) the directory with the given path.[7]

import os

path = os.getcwd()
directory = os.path.join(path, "__python_demo__")
if os.path.isdir(directory):
    raise Exception("Path already exists. Can't continue.")
os.mkdir(directory)
print("Created directory:", directory)
os.chdir(directory)
print("Changed to:", os.getcwd())
os.chdir(path)
print("Changed back to:", os.getcwd())
os.rmdir(directory)
print("Removed directory:", directory)

Output:

Created directory: /home/ubuntu/workspace/__python_demo__
Changed to: /home/ubuntu/workspace/__python_demo__
Changed back to: /home/ubuntu/workspace
Removed directory: /home/ubuntu/workspace/__python_demo__

The os.walk() Method

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The os.walk() method generates the subdirectories and files in a given path as a 3-tuple of a path string with subdirectory list and filename list.[8]

import os

for path, directories, files in os.walk(os.getcwd()):
    for directory in directories:
        print(os.path.join(path, directory))
    for file in files:
        print(os.path.join(path, file))

Output:

... <all subdirectories and files in the current working directory>

The os.path.isfile() Method

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The os.path.isfile() method returns True if the given path is an existing file.[9]

path = os.getcwd()
filename = os.path.join(path, "__python_demo.tmp")
if os.path.isfile(filename):
    print("File exists.")
else:
    print("File does not exist.")

Output:

File does not exist.

The open() Function

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The open() function opens the given file in the given mode (read, write, append) and returns a file object.[10]

file = open(filename, "r")    # read
file = open(filename, "w")    # write
file = open(filename, "a")    # append
file = open(filename, "r+")   # read + write
file = open(filename, "w+")   # read + write (new / cleared file)
file = open(filename, "a+")   # read + append (position starts at end of file)

The file.write() Method

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The file.write() method writes the contents of the given string to the file, returning the number of characters written.[11]

file.write("Temporary Python Demo File")

The file.close() Method

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The file.close() method closes the file and frees any system resources taken up by the open file.[12]

import os

path = os.getcwd()
filename = os.path.join(path, "__python_demo.tmp")
file = open(filename, "w")
file.write("Temporary Python Demo File")
file.close()
if os.path.isfile(filename):
    print("Created %s" % filename)

Output:

Created /home/ubuntu/workspace/__python_demo.tmp

The file.read() Method

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The file.read() method reads the given number of bytes from the file, or all content if no size is given, and returns the bytes that were read.[13]

import os

path = os.getcwd()
filename = os.path.join(path, "__python_demo.tmp")
if os.path.isfile(filename):
    file = open(filename, "r")
    text = file.read()
    file.close()
    print("File text:", text)

Output:

File text: Temporary Python Demo File

Reading Lines

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For reading lines from a file, you can loop over the file object. This is memory efficient, fast, and leads to simple code.[14]

import os

path = os.getcwd()
filename = os.path.join(path, "__python_demo.tmp")
file = open(filename, "r")
for line in file:
    print(line, end='')
file.close()

Output:

Temporary Python Demo File

The file.tell() Method

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The file.tell() method returns an integer giving the file object’s current position in the file.[15]

import os

path = os.getcwd()
filename = os.path.join(path, "__python_demo.tmp")
if os.path.isfile(filename):
    file = open(filename, "a+")
    print("Open file position:", file.tell())
    file.write(" - Appended to the end of the file")
    print("Write file position:", file.tell())

Output:

Open file position: 26
Write file position: 60

The file.seek() Method

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The file.seek() method moves the file position to the given offset from the given reference point. Reference points are 0 for the beginning of the file, 1 for the current position, and 2 for the end of the file.[16]

import os

path = os.getcwd()
filename = os.path.join(path, "__python_demo.tmp")
if os.path.isfile(filename):
    file = open(filename, "a+")
    print("Open file position:", file.tell())
    file.seek(0, 0)
    print("Seek file position:", file.tell())
    text = file.read()
    file.close()
    print("File text:", text)

Output:

Open file position: 60
Seek file position: 0
File text: Temporary Python Demo File - Appended to the end of the file

The os.rename() Method

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The os.rename() method renames the given source file or directory the given destination name.[17]

import os

path = os.getcwd()
filename = os.path.join(path, "__python_demo.tmp")
if not os.path.isfile(filename):
    raise Exception("File doesn't exist. Can't continue.")
filename2 = os.path.join(path, "__python_demo2.tmp")
if os.path.isfile(filename2):
    raise Exception("File already exists. Can't continue.")

os.rename(filename, filename2)
if os.path.isfile(filename2):
    print("Renamed %s to %s" % (filename, filename2))

Output:

Renamed /home/ubuntu/workspace/__python_demo.tmp to /home/ubuntu/workspace/__python_demo2.tmp

The os.remove() Method

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The os.remove() method removes (deletes) the given file.[18]

import os

path = os.getcwd()
filename2 = os.path.join(path, "__python_demo2.tmp")
if not os.path.isfile(filename2):
    raise Exception("File doesn't exist. Can't continue.")

os.remove(filename2)
if not os.path.isfile(filename2):
    print("Removed %s" % filename2)

Output:

Removed /home/ubuntu/workspace/__python_demo2.tmp

The sys.argv Property

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The sys.argv property returns the list of command line arguments passed to a Python script. argv[0] is the script name.[19]

import sys

for i in range(0, len(sys.argv)):
    print("sys.argv[%d]: %s" % (i, sys.argv[i]))

Output:

sys.argv[0]: /home/ubuntu/workspace/argv.py
sys.argv[1]: test1
sys.argv[2]: test2

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Practice

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  1. Create a Python program that displays high, low, and average quiz scores based on input from a file. Check for a filename parameter passed from the command line. If there is no parameter, ask the user to input a filename for processing. Verify that the file exists and then use RegEx methods to parse the file and add each score to a list. Display the list of entered scores sorted in descending order and then calculate and display the high, low, and average for the entered scores. Include error handling in case the file is formatted incorrectly. Create a text file of names and grade scores to use for testing based on the following format:
        Larry Fine: 80
        Curly Howard: 70
        Moe Howard: 90
  2. Create a Python program that asks the user for a file that contains HTML tags, such as:
        <p><strong>This is a bold paragraph.</strong></p>
    Check for a filename parameter passed from the command line. If there is no parameter, ask the user to input a filename for processing. Verify that the file exists and then use RegEx methods to search for and remove all HTML tags from the text, saving each removed tag in a dictionary. Print the untagged text and then use a function to display the list of removed tags sorted in alphabetical order and a histogram showing how many times each tag was used. Include error handling in case an HTML tag isn't entered correctly (an unmatched < or >). Use a user-defined function for the actual string processing, separate from input and output. For example:
        </p>: *
        </strong>: *
        <p>: *
        <strong>: *
  3. Create a Python program that asks the user for a file that contains lines of dictionary keys and values in the form:
        Larry Fine: 80
        Curly Howard: 70
        Moe Howard: 90
    Keys may contain spaces but should be unique. Values should always be an integer greater than or equal to zero. Check for a filename parameter passed from the command line. If there is no parameter, ask the user to input a filename for processing. Verify that the file exists and then use RegEx methods to parse the file and build a dictionary of key-value pairs. Then display the dictionary sorted in descending order by value (score). Include input validation and error handling in case the file accidentally contains the same key more than once.
  4. Create a Python program that checks all Python (.py) files in a given directory / folder. Check for a folder path parameter passed from the command line. If there is no parameter, ask the user to input a folder path for processing. Verify that the folder exists and then check all Python files in the folder for an initial docstring. If the file contains an initial docstring, continue processing with the next file. If the file does not start with a docstring, add a docstring to the beginning of the file similar to:
        """Filename.py"""
    Add a blank line between the docstring and the existing file code and save the file. Test the program carefully to be sure it doesn't alter any non-Python files and doesn't delete existing file content.

Lesson Summary

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File Concepts

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  • A file system is used to control how data is stored and retrieved. There are many different kinds of file systems. Each one has different structure and logic, properties of speed, flexibility, security, size and more.[20]
  • File systems are responsible for arranging storage space; reliability, efficiency, and tuning with regard to the physical storage medium are important design considerations.[21]
  • File systems allocate space in a granular manner, usually multiple physical units on the device.[22]
  • A filename (or file name) is used to identify a storage location in the file system.[23]
  • File systems typically have directories (also called folders) which allow the user to group files into separate collections.[24]
  • A file system stores all the metadata associated with the file—including the file name, the length of the contents of a file, and the location of the file in the folder hierarchy—separate from the contents of the file.[25]
  • Directory utilities may be used to create, rename and delete directory entries.[26]
  • File utilities create, list, copy, move and delete files, and alter metadata.[27]
  • All file systems have some functional limit that defines the maximum storable data capacity within that system.[28]
  • A directory is a file system cataloging structure which contains references to other computer files, and possibly other directories.[29]
  • A text file is a kind of computer file that is structured as a sequence of lines of electronic text.[30]
  • MS-DOS and Windows use a common text file format, with each line of text separated by a two-character combination: CR and LF, which have ASCII codes 13 and 10.[31]
  • Unix-like operating systems use a common text file format, with each line of text separated by a single newline character, normally LF.[32]

Python Files

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  • The os.getcwd() method returns a string representing the current working directory.[33]
  • The os.chdir() method changes the current working directory to the given path.[34]
  • The os.path.isdir() method returns True if the given path is an existing directory.[35]
  • The os.path.join() method joins one or more path components intelligently, avoiding extra directory separator (os.sep()) characters.[36]
  • The os.mkdir() method creates a directory with the given path.[37]
  • The os.rmdir() method removes (deletes) the directory with the given path.[38]
  • The os.walk() method generates the subdirectories and files in a given path as a 3-tuple of a path string with subdirectory list and filename list.[39]
  • The os.path.isfile() method returns True if the given path is an existing file.[40]
  • The open() function opens the given file in the given mode (read, write, append) and returns a file object.[41]
  • The file.write() method writes the contents of the given string to the file, returning the number of characters written.[42]
  • The file.close() method closes the file and frees any system resources taken up by the open file.[43]
  • The file.read() method reads the given number of bytes from the file, or all content if no size is given, and returns the bytes that were read.[44]
  • For reading lines from a file, you can loop over the file object using a for loop. This is memory efficient, fast, and leads to simple code.[45]
  • The file.tell() method returns an integer giving the file object’s current position in the file.[46]
  • The file.seek() method moves the file position to the given offset from the given reference point. Reference points are 0 for the beginning of the file, 1 for the current position, and 2 for the end of the file.[47]
  • The os.rename() method renames the given source file or directory the given destination name.[48]
  • The os.remove() method removes (deletes) the given file.[49]
  • The sys.argv property returns the list of command line arguments passed to a Python script. argv[0] is the script name.[50]
  • Python text mode file processing converts platform-specific line endings (\n on Unix, \r\n on Windows) to just \n on input and \n back to platform-specific line endings on output.[51]
  • Binary mode file processing must be used when reading and writing non-text files to prevent newline translation.[52]

Key Terms

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catch
To prevent an exception from terminating a program using the try and except statements.[53]
newline
A special character used in files and strings to indicate the end of a line.[54]
Pythonic
A technique that works elegantly in Python. “Using try and except is the Pythonic way to recover from missing files”.[55]
Quality Assurance
A person or team focused on insuring the overall quality of a software product. QA is often involved in testing a product and identifying problems before the product is released.[56]
text file
A sequence of characters stored in permanent storage like a hard drive.[57]

Review Questions

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Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. A file system is _____.
    A file system is used to control how data is stored and retrieved. There are many different kinds of file systems. Each one has different structure and logic, properties of speed, flexibility, security, size and more.
  2. File systems are responsible for _____.
    File systems are responsible for arranging storage space; reliability, efficiency, and tuning with regard to the physical storage medium are important design considerations.
  3. File systems allocate _____.
    File systems allocate space in a granular manner, usually multiple physical units on the device.
  4. A filename (or file name) is used to _____.
    A filename (or file name) is used to identify a storage location in the file system.
  5. File systems typically have directories (also called folders) which _____.
    File systems typically have directories (also called folders) which allow the user to group files into separate collections.
  6. A file system stores all the metadata associated with the file—including _____.
    A file system stores all the metadata associated with the file—including the file name, the length of the contents of a file, and the location of the file in the folder hierarchy—separate from the contents of the file.
  7. Directory utilities may be used to _____.
    Directory utilities may be used to create, rename and delete directory entries.
  8. File utilities _____.
    File utilities create, list, copy, move and delete files, and alter metadata.
  9. All file systems have some functional limit that defines _____.
    All file systems have some functional limit that defines the maximum storable data capacity within that system.
  10. A directory is _____.
    A directory is a file system cataloging structure which contains references to other computer files, and possibly other directories.
  11. A text file is _____.
    A text file is a kind of computer file that is structured as a sequence of lines of electronic text.
  12. MS-DOS and Windows use a common text file format, with _____.
    MS-DOS and Windows use a common text file format, with each line of text separated by a two-character combination: CR and LF, which have ASCII codes 13 and 10.
  13. Unix-like operating systems use a common text file format, with _____.
    Unix-like operating systems use a common text file format, with each line of text separated by a single newline character, normally LF.
  14. The os.getcwd() method _____.
    The os.getcwd() method returns a string representing the current working directory.
  15. The os.chdir() method _____.
    The os.chdir() method changes the current working directory to the given path.
  16. The os.path.isdir() method _____.
    The os.path.isdir() method returns True if the given path is an existing directory.
  17. The os.path.join() method _____.
    The os.path.join() method joins one or more path components intelligently, avoiding extra directory separator (os.sep()) characters.
  18. The os.mkdir() method _____.
    The os.mkdir() method creates a directory with the given path.
  19. The os.rmdir() method _____.
    The os.rmdir() method removes (deletes) the directory with the given path.
  20. The os.walk() method _____.
    The os.walk() method generates the subdirectories and files in a given path as a 3-tuple of a path string with subdirectory list and filename list.
  21. The os.path.isfile() method _____.
    The os.path.isfile() method returns True if the given path is an existing file.
  22. The open() function _____.
    The open() function opens the given file in the given mode (read, write, append) and returns a file object.
  23. The file.write() method _____.
    The file.write() method writes the contents of the given string to the file, returning the number of characters written.
  24. The file.close() method _____.
    The file.close() method closes the file and frees any system resources taken up by the open file.
  25. The file.read() method _____.
    The file.read() method reads the given number of bytes from the file, or all content if no size is given, and returns the bytes that were read.
  26. For reading lines from a file, you can _____.
    For reading lines from a file, you can loop over the file object using a for loop. This is memory efficient, fast, and leads to simple code.
  27. The file.tell() method _____.
    The file.tell() method returns an integer giving the file object’s current position in the file.
  28. The file.seek() method _____.
    The file.seek() method moves the file position to the given offset from the given reference point. Reference points are 0 for the beginning of the file, 1 for the current position, and 2 for the end of the file.
  29. The os.rename() method _____.
    The os.rename() method renames the given source file or directory the given destination name.
  30. The os.remove() method _____.
    The os.remove() method removes (deletes) the given file.
  31. The sys.argv property _____. argv[0] is _____.
    The sys.argv property returns the list of command line arguments passed to a Python script. argv[0] is the script name.
  32. Python text mode file processing _____.
    Python text mode file processing converts platform-specific line endings (\n on Unix, \r\n on Windows) to just \n on input and \n back to platform-specific line endings on output.
  33. Binary mode file processing must be used when _____.
    Binary mode file processing must be used when reading and writing non-text files to prevent newline translation.

Assessments

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See Also

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References

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  1. Vskills: Certified Python Developer
  2. Python.org: Miscellaneous operating system interfaces
  3. Python.org: Miscellaneous operating system interfaces
  4. Python.org: os.path
  5. Python.org: os.path
  6. Python.org: Miscellaneous operating system interfaces
  7. Python.org: Miscellaneous operating system interfaces
  8. Python.org: Miscellaneous operating system interfaces
  9. Python.org: os.path
  10. Python.org: Built-in Functions
  11. Python.org: Input and Output
  12. Python.org: Input and Output
  13. Python.org: Input and Output
  14. Python.org: Input and Output
  15. Python.org: Input and Output
  16. Python.org: Input and Output
  17. Python.org: Miscellaneous operating system interfaces
  18. Python.org: Miscellaneous operating system interfaces
  19. Python.org: System-specific parameters and functions
  20. Wikipedia: File system
  21. Wikipedia: File system
  22. Wikipedia: File system
  23. Wikipedia: File system
  24. Wikipedia: File system
  25. Wikipedia: File system
  26. Wikipedia: File system
  27. Wikipedia: File system
  28. Wikipedia: File system
  29. Wikipedia: Directory (computing)
  30. Wikipedia: Text file
  31. Wikipedia: Text file
  32. Wikipedia: Text file
  33. Python.org: Miscellaneous operating system interfaces
  34. Python.org: Miscellaneous operating system interfaces
  35. Python.org: os.path
  36. Python.org: os.path
  37. Python.org: Miscellaneous operating system interfaces
  38. Python.org: Miscellaneous operating system interfaces
  39. Python.org: Miscellaneous operating system interfaces
  40. Python.org: os.path
  41. Python.org: Built-in Functions
  42. Python.org: Input and Output
  43. Python.org: Input and Output
  44. Python.org: Input and Output
  45. Python.org: Input and Output
  46. Python.org: Input and Output
  47. Python.org: Input and Output
  48. Python.org: Miscellaneous operating system interfaces
  49. Python.org: Miscellaneous operating system interfaces
  50. Python.org: System-specific parameters and functions
  51. Python.org: Input and Output
  52. Python.org: Input and Output
  53. PythonLearn: Files
  54. PythonLearn: Files
  55. PythonLearn: Files
  56. PythonLearn: Files
  57. PythonLearn: Files

Lesson 14 - Internet Data

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This lesson introduces Python Internet-based data processing, including web pages and email (HTML, XML, JSON, and SMTP).


Objectives and Skills

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Objectives and skills for this lesson include:[1]

  • Standard Library
    • urllib and json modules

Readings

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  1. Wikipedia: HTML
  2. Wikipedia: XML
  3. Wikipedia: JSON
  4. Wikipedia: Simple Mail Transfer Protocol
  5. Python for Everyone: Networked programs
  6. Python for Everyone: Using Web Services

Multimedia

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  1. YouTube: Python for Informatics - Chapter 12 - Networked Programs
  2. YouTube Python for Informatics Chapter 13 - Web Services (Part 1/3)
  3. YouTube: Python for Informatics Chapter 13 - Web Services (Part 2/3)
  4. YouTube: Python for Informatics Chapter 13 - Web Services (Part 3/3)
  5. YouTube: Python - Downloading Files from the Web

Examples

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The urllib.request.urlopen() Method

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The urllib.request.urlopen() method opens the given URL. For HTTP and HTTPS URLs, it returns an http.client.HTTPResponse object that may be read like a file object.[2]

import urllib.request

url = "https://en.wikiversity.org/wiki/Python_Programming/Internet_Data"
try:
    page = urllib.request.urlopen(url).read().decode()
except Exception as exception:
    print(str(exception) + " reading " + url)
    exit(1)

for line in page.split("\n"):
    print(line)

Output:

<This page's source HTML...>

The xml.etree.ElementTree.fromstring() Method

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The xml.etree.ElementTree.fromstring() method parses XML from a string directly into an XML Element, which is the root element of the parsed tree.[3]

import xml.etree.ElementTree

root = xml.etree.ElementTree.fromstring(page)

The xml.etree.ElementTree.ElementTree() Method

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The xml.etree.ElementTree.ElementTree() method returns an ElementTree hierarchy for the given element.[4]

import xml.etree.ElementTree

tree = xml.etree.ElementTree.ElementTree(root)

The xml.etree.ElementTree.iter() Method

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The xml.etree.ElementTree.iter() method returns an iterator that loops over all elements in the tree, in section order.[5]

import urllib.request
import xml.etree.ElementTree

url = "http://www.w3schools.com/xml/note.xml"
try:
    page = urllib.request.urlopen(url).read()
    page = page.decode("UTF-8")
except Exception as exception:
    print(str(exception) + " reading " + url)
    exit(1)

root = xml.etree.ElementTree.fromstring(page)
tree = xml.etree.ElementTree.ElementTree(root)

for element in tree.iter():
    print("%s: %s" % (element.tag, element.text))

Output:

<The XML elements in the page http://www.w3schools.com/xml/note.xml...>

The xml.etree.ElementTree.findall() Method

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The xml.etree.ElementTree.findall() method finds only elements with a specific tag which are direct children of the current element.[6]

import urllib.request
import xml.etree.ElementTree

url = "http://www.w3schools.com/xml/note.xml"
try:
    page = urllib.request.urlopen(url).read()
    page = page.decode("UTF-8")
except Exception as exception:
    print(str(exception) + " reading " + url)
    exit(1)

root = xml.etree.ElementTree.fromstring(page)
tree = xml.etree.ElementTree.ElementTree(root)

for element in tree.findall("to"):
    print("%s: %s" % (element.tag, element.text))

Output:

<The XML "to" element(s) in the page http://www.w3schools.com/xml/note.xml...>

The json.loads() Method

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The json.loads() method converts a given JSON string to a corresponding Python object (dict, list, string, etc.).[7] The Wikimedia Pageview API is documented at https://wikitech.wikimedia.org/wiki/Analytics/PageviewAPI.

import urllib.request
import json

title = ""
url = "https://wikimedia.org/api/rest_v1/metrics/pageviews/per-article/en.wikiversity/all-access/user/" + \
    "Python_Programming%2fInternet_Data" + \
    "/daily/2016100100/2016103100"

try:
    page = urllib.request.urlopen(url).read().decode()
except Exception as exception:
    print(str(exception) + " reading " + url)
    exit(1)

print("Page Views")
dictionary = json.loads(page)
for item in dictionary["items"]:
    print("%s: %s" % (item["timestamp"], item["views"]))

Output:

<Page views for this page for 2016 October ...>

The smtplib Module

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The smtplib module defines an SMTP client session object that can be used to send mail to any Internet machine with an SMTP or ESMTP listener daemon.[8]

import smtplib
 
server = "smtp.gmail.com"
port = 587
username = "username"
password = "password"

sender = "me@domain"
recipient = "you@domain"
subject = "Python Email Test"
message = "Hello from Python!"

try:
    smtp = smtplib.SMTP(server, port)
    smtp.starttls()
    smtp.login(username, password)
    smtp.sendmail(sender, recipient, 
        "From: %s\nTo: %s\nSubject: %s\n%s" % (sender, recipient, subject, message))
    smtp.quit()

    print("Sent message.")
except Exception as exception:
    print(exception)

Output:

Sent message.

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Practice

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  1. Create a Python program that asks the user for a URL that contains HTML tags, such as:
        <p><strong>This is a bold paragraph.</strong></p>
    Check for a URL parameter passed from the command line. If there is no parameter, ask the user to input a URL for processing. Verify that the URL exists and then use RegEx methods to search for and remove all HTML tags from the text, saving each removed tag in a dictionary. Print the untagged text and then use a function to display the list of removed tags sorted in alphabetical order and a histogram showing how many times each tag was used. Include error handling in case an HTML tag isn't entered correctly (an unmatched < or >). Use a user-defined function for the actual string processing, separate from input and output. For example:
        </p>: *
        </strong>: *
        <p>: *
        <strong>: *
  2. Create a Python program that reads XML data from http://www.w3schools.com/xml/simple.xml and builds a list of menu items, with each list entry containing a dictionary with fields for the item's name, price, description, and calories. After parsing the XML data, display the menu items in decreasing order by price similar to:
        name - description - calories - price
  3. Create a Python program that asks the user for a location (city and state, province, or country). Use Google's Geocoding API to look up and display the given location's latitude and longitude. Also display a URL that could be used to pinpoint the given location on a map. The output should be similar to:
        Location: <location>
        Latitude: <latitude>
        Longitude: <longitude>
        Map: https://www.google.com/maps/@<latitude>,<longitude>,15z
  4. Create a Python program that asks the user for a Wikiversity page title and the user's email address. Check for URL and email address parameters passed from the command line. If there are no parameters, ask the user to input a page title and email address for processing. Verify that the Wikiversity page exists, and then check the page to see when it was last modified. If it was modified within the last 24 hours, send the user an email message letting them know that the page was modified recently. Include a link to the Wikiversity page in the email message.

Lesson Summary

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Internet Data Concepts

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  • HyperText Markup Language (HTML) is the standard markup language for creating web pages and web applications.[9]
  • HTML describes the structure of a web page semantically and originally included cues for the appearance (layout) of the document.[10]
  • HTML elements are the building blocks of HTML pages.[11]
  • HTML elements are delineated by tags, written using angle brackets.[12]
  • Tags are typically written using the syntax <tag>content</tag>. [13]
  • Some tags are written using the syntax <tag content />.[14]
  • Browsers do not display the HTML tags, but use them to interpret the content of the page.[15]
  • Cascading Style Sheets (CSS) define the look and layout of content.[16]
  • The HTML start tag may also include attributes within the tag, using the syntax <tag attribute="value" ... >content</tag>.[17]
  • The style attribute may be used to embed CSS style inside HTML tags using the syntax <tag style="property:value; ...">content</tag>.[18]
  • HTML comments are written using the syntax <!-- comment -->.[19]
  • Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable.[20]
  • Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary data structures such as those used in web services.[21]
  • Well-formed XML follows a syntax similar to HTML, using nested tags to represent data structure and values.[22]
  • JSON (JavaScript Object Notation) is an open-standard format that uses human-readable text to transmit data objects consisting of attribute–value pairs.[23]
  • JSON is the most common data format used for asynchronous browser/server communication, largely replacing XML.[24]
  • Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (email) transmission.[25]
  • Although electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically use SMTP only for sending messages to a mail server for relaying. For retrieving messages, client applications usually use either IMAP or POP3.[26]
  • SMTP communication between mail servers by default uses the TCP port 25. Mail clients often use port 587 to submit emails to the mail service. Despite being deprecated, the nonstandard port 465 is commonly used by mail providers.[27]
  • SMTP connections secured by SSL, known as SMTPS, can be made using STARTTLS.[28]

Python Internet Data

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  • The urllib.request.urlopen() method opens the given URL. For HTTP and HTTPS URLs, it returns an http.client.HTTPResponse object that may be read like a file object.[29]
  • The xml.etree.ElementTree.fromstring() method parses XML from a string directly into an XML Element, which is the root element of the parsed tree.[30]
  • The xml.etree.ElementTree.iter() method returns an iterator that loops over all elements in the tree, in section order.[31]
  • The json.loads() method converts a given JSON string to a corresponding Python object (dict, list, string, etc.).[32]
  • The smtplib module defines an SMTP client session object that can be used to send mail to any Internet machine with an SMTP or ESMTP listener daemon.[33]

Key Terms

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API
Application Program Interface - A contract between applications that defines the patterns of interaction between two application components.[34]
BeautifulSoup
A Python library for parsing HTML documents and extracting data from HTML documents that compensates for most of the imperfections in the HTML that browsers generally ignore. You can download the BeautifulSoup code from www.crummy.com.[35]
ElementTree
A built-in Python library used to parse XML data.[36]
JSON
JavaScript Object Notation. A format that allows for the markup of structured data based on the syntax of JavaScript Objects.[37]
port
A number that generally indicates which application you are contacting when you make a socket connection to a server. As an example, web traffic usually uses port 80 while email traffic uses port 25.[38]
scrape
When a program pretends to be a web browser and retrieves a web page, then looks at the web page content. Often programs are following the links in one page to find the next page so they can traverse a network of pages or a social network.[39]
SOA
Service-Oriented Architecture. When an application is made of components connected across a network.[40]
socket
A network connection between two applications where the applications can send and receive data in either direction.[41]
spider
The act of a web search engine retrieving a page and then all the pages linked from a page and so on until they have nearly all of the pages on the Internet which they use to build their search index.[42]
XML
eXtensible Markup Language. A format that allows for the markup of structured data.[43]

Review Questions

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Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. HyperText Markup Language (HTML) is _____.
    HyperText Markup Language (HTML) is the standard markup language for creating web pages and web applications.
  2. HTML describes _____.
    HTML describes the structure of a web page semantically and originally included cues for the appearance (layout) of the document.
  3. HTML elements are _____.
    HTML elements are the building blocks of HTML pages.
  4. HTML elements are delineated by _____.
    HTML elements are delineated by tags, written using angle brackets.
  5. Tags are typically written using the syntax _____. 
    Tags are typically written using the syntax <tag>content</tag>. 
  6. Some tags are written using the syntax _____.
    Some tags are written using the syntax <tag content />.
  7. Browsers do not display HTML tags, but use them _____.
    Browsers do not display HTML tags, but use them to interpret the content of the page.
  8. Cascading Style Sheets (CSS) define _____.
    Cascading Style Sheets (CSS) define the look and layout of content.
  9. The HTML start tag may also include _____.
    The HTML start tag may also include attributes within the tag, using the syntax <tag attribute="value" ... >content</tag>.
  10. The style attribute may be used to _____.
    The style attribute may be used to embed CSS style inside HTML tags using the syntax <tag style="property:value; ...">content</tag>.
  11. HTML comments are written using the syntax _____.
    HTML comments are written using the syntax <!-- comment -->.
  12. Extensible Markup Language (XML) is _____.
    Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable.
  13. Although the design of XML focuses on documents, the language is widely used for _____.
    Although the design of XML focuses on documents, the language is widely used for the representation of arbitrary data structures such as those used in web services.
  14. Well-formed XML follows a syntax similar to HTML, using _____.
    Well-formed XML follows a syntax similar to HTML, using nested tags to represent data structure and values.
  15. JSON (JavaScript Object Notation) is _____.
    JSON (JavaScript Object Notation) is an open-standard format that uses human-readable text to transmit data objects consisting of attribute–value pairs.
  16. JSON is the most common data format used for _____.
    JSON is the most common data format used for asynchronous browser/server communication, largely replacing XML.
  17. Simple Mail Transfer Protocol (SMTP) is _____.
    Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (email) transmission.
  18. Although electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically use _____.
    Although electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically use SMTP only for sending messages to a mail server for relaying. For retrieving messages, client applications usually use either IMAP or POP3.
  19. SMTP communication between mail servers by default uses TCP port _____. Mail clients often use port _____ to submit emails to the mail service. Despite being deprecated, the nonstandard port _____ is commonly used by mail providers.
    SMTP communication between mail servers by default uses the TCP port 25. Mail clients often use port 587 to submit emails to the mail service. Despite being deprecated, the nonstandard port 465 is commonly used by mail providers.
  20. SMTP connections secured by SSL, known as _____.
    SMTP connections secured by SSL, known as SMTPS, can be made using STARTTLS.
  21. The urllib.request.urlopen() method _____.
    The urllib.request.urlopen() method opens the given URL. For HTTP and HTTPS URLs, it returns an http.client.HTTPResponse object that may be read like a file object.
  22. The xml.etree.ElementTree.fromstring() method _____.
    The xml.etree.ElementTree.fromstring() method parses XML from a string directly into an XML Element, which is the root element of the parsed tree.
  23. The xml.etree.ElementTree.iter() method _____.
    The xml.etree.ElementTree.iter() method returns an iterator that loops over all elements in the tree, in section order.
  24. The json.loads() method _____.
    The json.loads() method converts a given JSON string to a corresponding Python object (dict, list, string, etc.).
  25. The smtplib module _____.
    The smtplib module defines an SMTP client session object that can be used to send mail to any Internet machine with an SMTP or ESMTP listener daemon.

Assessments

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See Also

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References

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Lesson 15 - Databases

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There are many different database implementations.This lesson introduces Python database processing using SQLite.


Objectives and Skills

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Objectives and skills for this lesson include:

  • Standard Library
    • sqlite3
    • logging module

Readings

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  1. Wikipedia: Database
  2. Wikipedia: SQL
  3. Wikipedia: SQLite
  4. Wikipedia: Logfile
  5. Python for Everyone: Using databases and SQL

Multimedia

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  1. YouTube: Creating a database, table, and inserting - SQLite3 with Python 3 part 1
  2. YouTube: Inserting variables to database table - SQLite3 with Python 3 part 2
  3. YouTube: Read from (SELECT) Database table - SQLite3 with Python 3 part 3

Examples

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Connecting to the database: The sqlite3.connect() Method

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The sqlite3.connect() method opens a connection to the given SQLite database file database.[1]

import sqlite3

connection = sqlite3.connect('users.db')

The connection.cursor() Method

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The connection.cursor() method creates and returns a cursor object that may be used to execute SQL commands.[2]

cursor = connection.cursor()

The cursor.execute() Method

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The cursor.execute() method executes an SQL statement.[3]

cursor.execute("""
    DROP TABLE IF EXISTS Users;
    """)

cursor.execute("""
    CREATE TABLE Users(
        UserID INT PRIMARY KEY NOT NULL,
        User TEXT NOT NULL
    );
    """)

The cursor.executescript() Method

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The cursor.executescript() method executes multiple SQL statements at once.[4]

cursor.executescript("""
    INSERT INTO Users(UserID, User) VALUES(1, 'Moe');
    INSERT INTO Users(UserID, User) VALUES(2, 'Larry');
    INSERT INTO Users(UserID, User) VALUES(3, 'Curly');
    """)

The cursor.fetchall() Method

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The cursor.fetchall() method fetches all (remaining) rows of a query result, returning a list.[5]

cursor.execute("""
    SELECT UserID, User FROM Users;
    """)
rows = cursor.fetchall()

The connection.commit() Method

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The connection.commit() method commits the current transaction.[6]

connection.commit()

The connection.close() Method

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The connection.close() method closes the database connection. Any changes that have not been committed will be lost.[7]

connection.close()

SQLite Database Example

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The following example uses sqlite3 to demonstrate database processing.

import sqlite3

connection = sqlite3.connect('users.db')
cursor = connection.cursor()

cursor.execute("""
    DROP TABLE IF EXISTS Users;
    """)

cursor.execute("""
    CREATE TABLE Users(
        UserID INT PRIMARY KEY NOT NULL,
        User TEXT NOT NULL
    );
    """)

cursor.executescript("""
    INSERT INTO Users(UserID, User) VALUES(1, 'Moe');
    INSERT INTO Users(UserID, User) VALUES(2, 'Larry');
    INSERT INTO Users(UserID, User) VALUES(3, 'Curly');
    """)

cursor.execute("""
    SELECT UserID, User FROM Users;
    """)
rows = cursor.fetchall()

connection.commit()
connection.close()

Logging to the Console

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The logging module includes methods that provide a means of tracking events that happen when some software runs. Events have a descriptive message and an importance which the developer ascribes to the event; the importance can also be called the level or severity. Python includes severity levels for DEBUG, INFO, WARNING, ERROR, and CRITICAL. By default, logging messages are sent to the console.[8]

import logging

# Log all severity levels.
logging.basicConfig(level=logging.DEBUG)

logging.debug("Debug message sent to console.")
logging.info("Info message sent to console.")
logging.warning("Warning message sent to console.")
logging.error("Error message sent to console.")
logging.critical("Critical message sent to console.")

Output:

DEBUG:root:Debug message sent to console.
INFO:root:Info message sent to console.
WARNING:root:Warning message sent to console.
ERROR:root:Error message sent to console.
CRITICAL:root:Critical message sent to console.

Logging to a File

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The logging.basicConfig() method can be used to configure logging to output to a file.[9]

import logging

# Log all severity levels to test.log
logging.basicConfig(filename='test.log', level=logging.DEBUG)

logging.debug("Debug message sent to file.")
logging.info("Info message sent to file.")
logging.warning("Warning message sent to file.")
logging.error("Error message sent to file.")
logging.critical("Critical message sent to file.")

Output sent to test.log:

DEBUG:root:Debug message sent to file.
INFO:root:Info message sent to file.
WARNING:root:Warning message sent to file.
ERROR:root:Error message sent to file.
CRITICAL:root:Critical message sent to file.

Activities

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Tutorials

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  1. Complete one or more of the following tutorials:

Practice

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  1. Create a Python program that saves data from a text file in an SQLite database. Check for a filename parameter passed from the command line. If there is no parameter, ask the user to input a filename for processing. Verify that the file exists and then use RegEx methods to parse the file and add each name and score to the database. Query the database to display the stored data in descending order by score. Include error handling in case the file is formatted incorrectly. Create a text file of names and grade scores to use for testing based on the following format:
        Larry Fine: 80
        Curly Howard: 70
        Moe Howard: 90
  2. Create a Python program that reads XML data from http://www.w3schools.com/xml/simple.xml and saves the menu items in an SQLite database. Include fields for the item's name, price, description, and calories. After parsing and saving the XML data, query the database and display the menu items in decreasing order by price similar to:
        name - description - calories - price
  3. Create a Python program that asks the user for a Wikiversity page title. Use the Wikimedia Pageview API to look up page view statistics for the page for the current month and save the statistics in an SQLite database. Include fields for the page title, date, and page views. After saving the JSON data, query the database and display the page view statistics in decreasing order by page views.

Lesson Summary

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Database Concepts

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  • A database is an organized collection of data.[10]
  • A database is a collection of schemas, tables, queries, reports, views and other objects.[11]
  • A query language is a computer language used to make queries (or questions about data) in databases and information systems.[12]
  • A database management system (DBMS) is a computer software application that interacts with the user, other applications, and the database itself to capture and analyze data. A general-purpose DBMS is designed to allow the definition, creation, querying, update, and administration of databases.[13]
  • A database table is a collection of related data held in a structured format within a database consisting of fields (columns), and rows.[14]
  • A data manipulation language (DML) is a family of syntax elements similar to a computer programming language used for selecting, inserting, deleting and updating data in a database.[15]
  • A data definition language (DDL) is a syntax similar to a computer programming language for defining data structures. Structured query language uses a collection of verbs used to modify the schema of the database by adding, changing, or deleting definitions of tables or other objects.[16]
  • A SELECT statement retrieves zero or more rows from one or more database tables or database views.[17]
  • An INSERT statement adds one or more records to any single table in a relational database.[18]
  • An UPDATE statement changes the data of one or more records in a table. Either all the rows can be updated, or a subset may be chosen using a condition.[19]
  • A DELETE statement removes one or more records from a table.[20]
  • SQLite is a relational database management system contained in a C programming library. In contrast to many other database management systems, SQLite is not a client–server database engine. Rather, it is embedded into the end program.[21]

Python Databases

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  • The sqlite3.connect() method opens a connection to the given SQLite database file database.[22]
  • The connection.cursor() method creates and returns a cursor object that may be used to execute SQL commands.[23]
  • The cursor.execute() method executes an SQL statement.[24]
  • The cursor.executescript() method executes multiple SQL statements at once.[25]
  • The cursor.fetchall() method fetches all (remaining) rows of a query result, returning a list.[26]
  • The connection.commit() method commits the current transaction.[27]
  • The connection.close() method closes the database connection. Any changes that have not been committed will be lost.[28]
  • The Python logging module includes methods that provide a means of tracking events that happen when some software runs. Events have a descriptive message and an importance which the developer ascribes to the event; the importance can also be called the level or severity. Python includes severity levels for DEBUG, INFO, WARNING, ERROR, and CRITICAL. By default, logging messages are sent to the console.[29]
  • The logging.basicConfig() method can be used to configure logging to output to a file.[30]

Key Terms

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attribute
One of the values within a tuple. More commonly called a “column” or “field”.[31]
constraint
When we tell the database to enforce a rule on a field or a row in a table. A common constraint is to insist that there can be no duplicate values in a particular field (i.e., all the values must be unique).[32]
cursor
A cursor allows you to execute SQL commands in a database and retrieve data from the database. A cursor is similar to a socket or file handle for network connections and files, respectively.[33]
database browser
A piece of software that allows you to directly connect to a database and manipulate the database directly without writing a program.[34]
foreign key
A numeric key that points to the primary key of a row in another table. Foreign keys establish relationships between rows stored in different tables.[35]
index
Additional data that the database software maintains as rows and inserts into a table to make lookups very fast.[36]
logical key
A key that the “outside world” uses to look up a particular row. For example in a table of user accounts, a person’s email address might be a good candidate as the logical key for the user’s data.[37]
normalization
Designing a data model so that no data is replicated. We store each item of data at one place in the database and reference it elsewhere using a foreign key.[38]
primary key
A numeric key assigned to each row that is used to refer to one row in a table from another table. Often the database is configured to automatically assign primary keys as rows are inserted.[39]
relation
An area within a database that contains tuples and attributes. More typically called a “table”.[40]
tuple
A single entry in a database table that is a set of attributes. More typically called “row”.[41]

Review Questions

edit
Enable JavaScript to hide answers.
Click on a question to see the answer.
  1. A database is _____.
    A database is an organized collection of data.
  2. A database is a collection of _____, _____, _____, _____, _____ and _____.
    A database is a collection of schemas, tables, queries, reports, views and other objects.
  3. A query language is _____.
    A query language is a computer language used to make queries (or questions about data) in databases and information systems.
  4. A database management system (DBMS) is _____.
    A database management system (DBMS) is a computer software application that interacts with the user, other applications, and the database itself to capture and analyze data. A general-purpose DBMS is designed to allow the definition, creation, querying, update, and administration of databases.
  5. A database table is _____.
    A database table is a collection of related data held in a structured format within a database consisting of fields (columns), and rows.
  6. A data manipulation language (DML) is _____.
    A data manipulation language (DML) is a family of syntax elements similar to a computer programming language used for selecting, inserting, deleting and updating data in a database.
  7. A data definition language (DDL) is _____.
    A data definition language (DDL) is a syntax similar to a computer programming language for defining data structures. Structured query language uses a collection of verbs used to modify the schema of the database by adding, changing, or deleting definitions of tables or other objects.
  8. A SELECT statement _____.
    A SELECT statement retrieves zero or more rows from one or more database tables or database views.
  9. An INSERT statement _____.
    An INSERT statement adds one or more records to any single table in a relational database.
  10. An UPDATE statement _____.
    An UPDATE statement changes the data of one or more records in a table. Either all the rows can be updated, or a subset may be chosen using a condition.
  11. A DELETE statement _____.
    A DELETE statement removes one or more records from a table.
  12. SQLite is _____.
    SQLite is a relational database management system contained in a C programming library. In contrast to many other database management systems, SQLite is not a client–server database engine. Rather, it is embedded into the end program.
  13. The sqlite3.connect() method _____.
    The sqlite3.connect() method opens a connection to the given SQLite database file database.
  14. The connection.cursor() method _____.
    The connection.cursor() method creates and returns a cursor object that may be used to execute SQL commands.
  15. The cursor.execute() method _____.
    The cursor.execute() method executes an SQL statement.
  16. The cursor.executescript() method _____.
    The cursor.executescript() method executes multiple SQL statements at once.
  17. The cursor.fetchall() method _____.
    The cursor.fetchall() method fetches all (remaining) rows of a query result, returning a list.
  18. The connection.commit() method _____.
    The connection.commit() method commits the current transaction.
  19. The connection.close() method _____.
    The connection.close() method closes the database connection. Any changes that have not been committed will be lost.
  20. The Python logging module .
    The Python logging module includes methods that provide a means of tracking events that happen when some software runs. Events have a descriptive message and an importance which the developer ascribes to the event; the importance can also be called the level or severity. Python includes severity levels for DEBUG, INFO, WARNING, ERROR, and CRITICAL. By default, logging messages are sent to the console.
  21. The logging.basicConfig() method can be used to _____.
    The logging.basicConfig() method can be used to configure logging to output to a file.

Assessments

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See Also

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References

edit
  1. Python.org: sqlite3
  2. Python.org: sqlite3
  3. Python.org: sqlite3
  4. Python.org: sqlite3
  5. Python.org: sqlite3
  6. Python.org: sqlite3
  7. Python.org: sqlite3
  8. Python.org: Logging How To
  9. Python.org: Logging How To
  10. Wikipedia: Database
  11. Wikipedia: Database
  12. Wikipedia: Query language
  13. Wikipedia: Database
  14. Wikipedia: Table (database)
  15. Wikipedia: Data manipulation language
  16. Wikipedia: Data definition language
  17. Wikipedia: Select (SQL)
  18. Wikipedia: Insert (SQL)
  19. Wikipedia: Update (SQL)
  20. Wikipedia: Delete (SQL)
  21. Wikipedia: SQLite
  22. Python.org: sqlite3
  23. Python.org: sqlite3
  24. Python.org: sqlite3
  25. Python.org: sqlite3
  26. Python.org: sqlite3
  27. Python.org: sqlite3
  28. Python.org: sqlite3
  29. Python.org: Logging How To
  30. Python.org: Logging How To
  31. PythonLearn: Using databases and Structured Query Language (SQL)
  32. PythonLearn: Using databases and Structured Query Language (SQL)
  33. PythonLearn: Using databases and Structured Query Language (SQL)
  34. PythonLearn: Using databases and Structured Query Language (SQL)
  35. PythonLearn: Using databases and Structured Query Language (SQL)
  36. PythonLearn: Using databases and Structured Query Language (SQL)
  37. PythonLearn: Using databases and Structured Query Language (SQL)
  38. PythonLearn: Using databases and Structured Query Language (SQL)
  39. PythonLearn: Using databases and Structured Query Language (SQL)
  40. PythonLearn: Using databases and Structured Query Language (SQL)
  41. PythonLearn: Using databases and Structured Query Language (SQL)

Lesson 16 - GUI

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This lesson introduces Python graphical user interfaces using Tkinter.


Objectives and Skills

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Objectives and skills for this lesson include:

  • Standard Library
    • Graphical User Interfaces

Readings

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  1. Wikipedia: Graphical user interface
  2. Wikipedia: Platform-independent GUI library
  3. Wikipedia: Tkinter

Multimedia

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  1. YouTube: TkInter Tutorial
  2. YouTube: Python tkinter Playlist

Examples

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The tkinter.Tk() Method

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The tkinter.Tk() method creates a toplevel widget of Tk, which usually is the main window of an application.[1]

import tkinter

root = tkinter.Tk()

root.mainloop()

The root.title() Method

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The root.title() method sets the window title.[2]

root.title("tkinter Example")

The root.maxsize() Method

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The maxsize() method returns the maximum size available for the window.[3]

root.maxsize()

The root.geometry() Method

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The geometry() method sets the size and offset in pixels for the window.[4]

root.geometry("%dx%d+0+0" % root.maxsize())

The tkinter.scrolledtext Object

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The tkinter.scrolledtext object implements a basic text widget with a vertical scroll bar.[5]

import tkinter.scrolledtext

scrolledtext = tkinter.scrolledtext.ScrolledText(root)

The pack() Method

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The pack() method packs widgets into the available window space.[6]

scrolledtext.pack(fill="both", expand=True)

The scrolledtext.insert() Method