# Applied Programming/Conditions/Python3

## conditions.py

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

Input:
Fahrenheit temperature

Output:
Fahrenheit temperature
Celsius temperature

Example:
Enter Fahrenheit temperature:
100
100.0° Fahrenheit is 37.8° Celsius

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

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

"""
try:
print("Enter Fahrenheit temperature:")
fahrenheit = input()
fahrenheit = float(fahrenheit)
if fahrenheit < ABSOLUTE_ZERO:
print("Fahrenheit temperature cannot be below absolute zero.")
print(f"ValueError: {fahrenheit} is invalid."
os._exit(1)
return fahrenheit
except ValueError:
print("Fahrenheit temperature must be a floating point value.")
print(f"ValueError: {fahrenheit} is invalid.")
os._exit(2)

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(f"fahrenheit must be a float. Received '{fahrenheit}'")
except:
raise

if fahrenheit < ABSOLUTE_ZERO:
raise ValueError(f"fahrenheit must not be below absolute zero. Received '{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 {type(fahrenheit)}"
assert isinstance(celsius, float) or isinstance(celsius, int), \
"celsius must be a float. Received {type(celsius)}"
print(f"{fahrenheit}.1f° Fahrenheit is {celsius}.1f° Celsius")

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

try:
fahrenheit = get_fahrenheit()
celsius = fahrenheit_to_celsius(fahrenheit)
display_results(fahrenheit, celsius)
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.