Why does my recursive function return None?

To return a value other than None, you need to use a return statement.

In your case, the if block only executes a return when executing one branch. Either move the return outside of the if/else block, or have returns in both options.

It is returning None because when you recursively call it:

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

..you don't return the value.

So while the recursion does happen, the return value gets discarded, and then you fall off the end of the function. Falling off the end of the function means that python implicitly returns None, just like this:

>>> def f(x):
...     pass
>>> print(f(20))
None

So, instead of just calling get_input() in your if statement, you need to return it:

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

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.')
        return get_input()
    else:
        return my_var

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