Python Why does the fucntion return None instead of False

The control statement doesn't reach

    result = True
    return result

So the function has nothing to return, so it returns None.

This will return either True or False

def func(l1, l2):
    result = False
    for x in l1:
        for y in l2:
            if x == y:
                result = True
    return result

May I suggest a much cleaner (and faster) solution? One of Python's strengths is list comprehension.

def func(l1, l2):
    return any([x==y for x in l1 for y in l2])

print(func([1,2,3,4,5], [5,6,7,8,9]))
print(func([1,2,3,4,5], [6,7,8,9])) 

Output:

True
False

As your function is currently written, it only works when there is a pairwise match between at least one of the elements of l1 and l2 - it would correctly set result from False to True, then return the result. However, if none of the elements in l1 and l2 are pairwise equal, the return statement is never reached because your function will exit the loop and then terminate the function - without returning anything.

Therefore, the return statement needs to be outside of the loop. Otherwise, your function won't return anything if none of the elements in the lists are equal.

def func(l1, l2):
    result = False
    for x in l1:
        for y in l2:
            if x == y:
                result = True
    return result

So let's walk through what happens here. You check each element of l1 and l2 and if any of them are equal you return something. For now, just leave it as something. Which means in this piece of code-

if x == y:
    result = True
    return result

You only enter do result = True and return result statement if x == y. If there is no scenario where x == y occurs as it happens in the second example you mentioned. There is a variable called result = False but there is no return statement triggered. Which means it does not return anything. So, you get a None.

Right way to do this

You can make this edit in your code to make it work like you asked for-

def func(l1, l2):
    result = False
    for x in l1:
        for y in l2:
            if x == y:
                result = True
                return result
    return result

What this does is instead of returning the None if the inner return statement is not triggered it returns result which would be False.