How Can I Prevent Recursive Functions from Returning None in Python?

Resolving the None Return Problem in Recursive Functions

When dealing with recursive functions, ensuring proper return handling is crucial to avoid unexpected None values. Consider the following function that repeatedly prompts users to enter either "a" or "b":

def get_input():
    my_var = input('Enter "a" or "b": ')

    if my_var != "a" and my_var != "b":
        print('You didn\'t type "a" or "b". Try again.')
        get_input()
    else:
        return my_var

print('got input:', get_input())

When users correctly input "a" or "b," the function works as expected, but if they enter any other character, the function prints an error message and recursively calls itself. However, the recursive call is not correctly returning the collected input.

The problem arises from the recursive call not being terminated with a return statement when the input is not "a" or "b." Python interprets this as reaching the end of the function and implicitly returns None. This behavior is similar to:

def f(x):
    pass
print(f(20))  # Implicitly returns None

To fix this issue, one must modify the if statement to return the result of the recursive get_input() call:

if my_var != "a" and my_var != "b":
    print('You didn\'t type "a" or "b". Try again.')
    return get_input()

By incorporating this adjustment, the function correctly returns the input even when users input incorrect characters and then correct their input.

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