Documentation

Empirically, it is the case that when execution reaches the end of a user-defined function body without encountering a return statement, the function returns None.

def foo():
    pass
    # implicit `return None`

And the docs hint at this in places! From here, in 6.3.4, emphasis added:

A call always returns some value, possibly None, unless it raises an exception. How this value is computed depends on the type of the callable object.

Later, in User-Defined Functions:

When the code block executes a return statement, this specifies the return value of the function call.

Additionally, here in 3.2:

[None] is used to signify the absence of a value in many situations, e.g., it is returned from functions that don’t explicitly return anything.

But the reference doesn't actually come out and explicitly say what happens in the absence of a return statement.

Possible Fix:

Ideally, I'd add one more sentence to the User-Defined functions section after the above quote:

If the code block reaches the end of the function body without executing a return statement, the return value is None.