Logic behind Pylint error E1128 (assignment-from-none)

Question

Consider the following use case (minimum example):

def get_func(param):

    if param is None:
        def func():
            return None
    else:
        def func():
            return param

    return func


def process_val(param):

    func = get_func(param)
    val = func()

    # Do stuff with 'val'; *None* is an useful case.
    return val

Here, func() can return None or not, depending on the value of param, but Pylint triggers E1128 for this, with the following explanation:

Used when an assignment is done on a function call but the inferred function returns nothing but None.

I am tempted to just disable the warning for this code, but it is actually classified as an Error, which makes me think this has actually produced bugs in the past, so I would like to understand: is this a Pylint error, that doesn't see that sometimes the function created will return something else than None? Or is it considered too bad practice to possibly have a function that always returns None? Maybe some other explanation that I cannot see?

In case this seems like a too convoluted, the actual use case is more like this:

def get_func(source):

    if source is None:
        def func():
            return None
   
    elif source is "webcam":
        # Open webcam...
        def func():
            # Capture frame from webcam
            return frame
   
    elif source is "server":
        # Open connection to server...
        def func():
            # Read data from server.
            return data

   # Other cases...

    return func


def process_val(source):

    data_func = get_func(source)

    # Here, do stuff in a loop, or pass *data_func* to other functions...
    # The code that uses the *data_func* knows that *None* means that
    # data could not be read and that's OK.

For the code that uses data_func, it's simpler like this than to having to consider the value of source to decide if the data will always be None. To me this seems a valid functional-style approach (maybe I'm wrong and this is not the Pythonic way).

(I'm using Pylint 2.12.2)

Answer 1

If the function does not always return None, then it's a false positive from pylint not understanding your code well. If the function always return None you have no reason to assign it in a variable, and it means the code is at best is doing a useless assignment, not doing what you think it does, or at worst completely wrong. Not sure why it's an error message and not a warning though.