Why does my recursive function return None?

Question

I have this function that calls itself:

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())

Now, if I input just "a" or "b", everything works fine:

Type "a" or "b": a
got input: a

But, if I type something else and then "a" or "b", I get this:

Type "a" or "b": purple
You didn't type "a" or "b". Try again.
Type "a" or "b": a
got input: None

I don't know why get_input() is returning None since it should only return my_var. Where is this None coming from and how do I fix my function?

Rackley

You need to do return Dat_Function() when calling it recursively. – Jemy

Just a tip: The idiomatic way of that my_var != "a" and my_var != "b" condition would be my_var not in ('a', 'b') – Breakup

@Breakup not necessarily. Now you're hitting the heap allocating a tuple just to do a simple comparison. Could be painful in a critical path and it's not much more readable, really. – Dulcedulcea

This is a simple example of recursion for demonstration purposes; but in case you actually need to do this task, a while loop makes more sense. See Asking the user for input until they give a valid response. – Readytowear

Sometimes, people run into this problem when trying to combine iteration and recursion. If you have a recursive call inside a loop, it might not be clear what to do with the result - since return would break out of the loop. In general, however, this is the same problem as if you were trying to call any other function, rather than using recursion. It is also a commonly asked quesiton, with a reference duplicate here: How can I use return to get back multiple values from a loop? Can I put them in a list? – Readytowear

Answer 1

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 what the recursive call returns:

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

Snowclad

Shouldn't it run through the if statement again if it is called recursively? I don't understand why it wouldn't return a value. – Rackley

Nope. See my edit. The recursion happens, and then you discard what the recursion returns. – Snowclad

So if you call a function from inside that same function the return value gets discarded, but you return the same function in that function you really just call it in main()? – Rackley

You lost me with that main() bit... You can fail as many times as you want to, the one that "succeeds" will return my_var, which will get passed down (returned) through all of the recursive calls all the way down to the original caller. Which, yes, is main(). – Snowclad

I was thinking that when you return Dat_Function() you're really just calling Dat_Function() again in main(). Dat_Function() now returns a function and main() has go call it. – Rackley

Use return for recursive function in order to put its value into the stack , so that when function will do recursion values from the stack are taken one by one. If you don't use return , the stack will collect only "None" values. – Leader

you, sir, are a genius! That was not intuitive to me. – Merocrine

Answer 2

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.

Pickering

I've tried moving it out of the block, but to no avail. Instead of returning the correct value, it returns the first incorrect value. Also, I don't want a return statement for the if part of the if/else statement because I want the function to only return a correct value. – Rackley

This language "feature" tripped me up for days. As a functional programmer I find it very counterintuitive – Successor

Answer 3

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())

Blaineblainey

Answer 4

I think to understand more what actually going on in your recursive function you should try to debug your code. There is interesting visualizing code execution tool that I recommend called Python Turor.

I will try this test case on your recursive function and visualize the execution process:

First input my_var as x then enter my_var as a.

You can see from the debugging visualizer that when my_var = a it will execute the return statement.

Then the recursive function will return the input value a at this line of code in the recursive function.

After that it will go to execute get_input() function again which it doesn't return any value that's the reason the final value of print('got input:', get_input()) is None.

If your replace get_input() call inside recursive function with return get_input() It will return my_var value which is a in this test case.

Hope this demonstration using Python Tutor debugging visualization tool would be helpful in clarifying the execution process of recursive function.

Saimon