# Applied Programming/Loops/Python3

## loops.py

```"""This program converts a Fahrenheit temperature to Celsius.

Input:
Fahrenheit temperature

Output:
Fahrenheit temperature
Celsius temperature

Example:
Enter Fahrenheit temperature or press <Enter> to quit:
100
100.0° Fahrenheit is 37.8° Celsius

Enter Fahrenheit temperature or press <Enter> to quit:
...

References:
* https://wiki.python.org/moin/UsingAssertionsEffectively
* https://stackoverflow.com/questions/1278705/python-when-i-catch-an-exception-how-do-i-get-the-type-file-and-line-number
* http://www.mathsisfun.com/temperature-conversion.html

"""

import os
import sys

ABSOLUTE_ZERO = -459.67

def get_fahrenheit():
"""Gets Fahrenheit temperature.

Args:
None

Returns:
float: Fahrenheit temperature or None if no temperature entered.

"""
while True:
print("Enter Fahrenheit temperature or press <Enter> to quit:")
fahrenheit = input()
try:
fahrenheit = float(fahrenheit)
if fahrenheit < ABSOLUTE_ZERO:
print("Fahrenheit temperature cannot be below absolute zero.")
print("ValueError: '%f' is invalid.\n" % fahrenheit)
else:
return fahrenheit
except ValueError:
if fahrenheit == '':
return None
else:
print("Fahrenheit temperature must be a floating point value.")
print("ValueError: '%s' is invalid.\n" % fahrenheit)

def fahrenheit_to_celsius(fahrenheit):
"""Converts Fahrenheit temperature to Celsius.

Args:
fahrenheit (float): Fahrenheit temperature to be converted

Returns:
float: Celsius temperature

Raises:
ValueError: If Fahrenheit temperature is not a valid float.
ValueError: If Fahrenheit temperature is below absolute zero.

"""
try:
fahrenheit = float(fahrenheit)
except ValueError:
raise ValueError("fahrenheit must be a float. Received '" + str(fahrenheit) + "'")
except:
raise

if fahrenheit < ABSOLUTE_ZERO:
raise ValueError(
"fahrenheit must not be below absolute zero. Received %f" % fahrenheit)

TEMPERATURE_DIFFERENCE = 32
TEMPERATURE_RATIO = 5 / 9
celsius = (fahrenheit - TEMPERATURE_DIFFERENCE) * TEMPERATURE_RATIO
return celsius

def display_results(fahrenheit, celsius):
"""Displays Fahrenheit and Celsius temperatures.

Args:
fahrenheit (float): Fahrenheit temperature
celsuis (float): Celsius temperature

Returns:
None

Raises:
AssertionError: If Fahrenheit temperature is not a valid float.
AssertionError: If Celsius temperature is not a valid float.

"""
assert isinstance(fahrenheit, float) or isinstance(fahrenheit, int), \
"fahrenheit must be a float. Received %s" % type(fahrenheit)
assert isinstance(celsius, float) or isinstance(celsius, int), \
"celsius must be a float. Received %s" % type(celsius)
print("%.1f° Fahrenheit is %.1f° Celsius\n" % (fahrenheit, celsius))

def display_table(fahrenheit):
"""Displays nearest multiples of 10 Fahrenheit and Celsius temperatures.

Args:
fahrenheit (float): Fahrenheit temperature

Returns:
None

Raises:
AssertionError: If Fahrenheit temperature is not a valid float.

"""
assert isinstance(fahrenheit, float) or isinstance(fahrenheit, int), \
"fahrenheit must be a float. Received %s" % type(fahrenheit)

print("F\tC")
start = int(fahrenheit / 10) * 10
for fahrenheit in range(start, start + 11):
print("%.1f" % fahrenheit, end="\t")
print("%.1f" % fahrenheit_to_celsius(fahrenheit))
print()

def main():
"""Runs the main program logic."""

while True:
try:
fahrenheit = get_fahrenheit()
if fahrenheit == None:
break
celsius = fahrenheit_to_celsius(fahrenheit)
display_results(fahrenheit, celsius)
display_table(fahrenheit)
except:
print("Unexpected error.")
print("Error:", sys.exc_info()[1])
print("File: ", sys.exc_info()[2].tb_frame.f_code.co_filename)
print("Line: ", sys.exc_info()[2].tb_lineno)

main()
```

## Try It

Copy and paste the code above into one of the following free online development environments or use your own Python3 compiler / interpreter / IDE.