None

Consider this function called best_animal():

def best_animal() -> str:
    response = input('What do you think is the best animal? ')
    return response


if __name__ == '__main__':
    animal = best_animal()

If a person enters ‘giraffe’ at the prompt, then response → 'giraffe', and after the function returns animal → 'giraffe'.

However, what happens if you leave off the return statement?

def best_animal():
    response = input('What do you think is the best animal? ')


if __name__ == '__main__':
    animal = best_animal()

In this case, the best_animal() function returns None! So then animal → None.

Using None

Having a value that represents None, or nothing, can be really useful. For example, look at this revised best_animal() function:

def best_animal() -> str | None:
    response = input('What do you think is the best animal? ')
    if response == '':
        return None

    return response


if __name__ == '__main__':
    animal = best_animal()
    if animal is None:
        print("You don't have a favorite animal? 😔")
    else:
        print(f"Your favorite animal is a {animal}? Cool!")

You can find this code in best_animals.py in the zip file above. Walk through this program with the debugger to see what happens in these cases:

• If you enter ‘giraffe’, then:

  • The variable response → 'giraffe'.
  • Since this is not equal to an empty string, then best_animal() returns ‘giraffe’.
  • This sets the variable animal → 'giraffe'.
  • Since animal is not None, the program prints “Your favorite animal is a giraffe? Cool!”

• If you enter nothing, by just pressing the Enter key, then:

  • The variable response → ''.
  • Since response is equal to the empty string, then best_animal() returns None.
  • This sets the variable animal → None.
  • Since animal is None, the program prints “You don’t have a favorite animal? 😔”

You can check if a variable is None using is None:

if animal is None:
    # do something

Likewise, you can check if a variable is not None using is not None`:

if animal is not None:
    # do something