--warn-return-any should not complain on NotImplemented when using abc

[drtyrsa]

I'm using abc.abstractmethod:

@abc.abstractmethod
def encode(self) -> bytes:
    return NotImplemented

And I get a warning: warning: Returning Any from function declared to return "bytes"

I don't think something is wrong with the code, it's totally normal way to declare and describe an interface and mypy shouldn't complain about it.

[ilevkivskyi]

Why do you think this is a standard pattern? I always thought that NotImplemented is reserved for reversed operators (like __radd__ vs __add__) and few other special methods (like __subclasshook__). I think the standard way of doing what you want is:

@abc.abstractmethod
def encode(self) -> bytes:
    raise NotImplementedError("Not a valid encoder -- implement `encode` before use")

and I hope mypy doesn't complain about this (if yes, then this should be fixed).

[gvanrossum]

You have a point there. I wonder if the correct type for `NotImplemented` shouldn't be `object` instead of `Any`. Can you try this (it's a one-line change in typeshed) and submit a PR?

[ilevkivskyi]

@gvanrossum It is not possible because NotImplemented is actually used in many __op__ methods that return non-Any types. All such cases will be flagged as error by mypy if we type NotImplemented as object.

[ilevkivskyi]

A simplest example is __eq__, it is required to return bool, and therefore a standard pattern like:

def __eq__(self, other: object) -> bool:
    if isinstance(other, type(self)):
        ...
    return NotImplemented

will be flagged as an error.

[gvanrossum]

Right. Maybe --disallow-any-expr can be hacked to ignore this case?

[ilevkivskyi]

Maybe --disallow-any-expr can be hacked to ignore this case?

Yes, this is easy, but would not be my priority TBH, if OP wants to make a PR then OK.

[drtyrsa]

@ilevkivskyi I've reread the docs and you are right, I shouldn't use NotImplemented here. I'll make a PR about return NotImplemented case.

Strangely abc docs doesn't say anything about what should a body of an abstract method look like. PEP 3119 doesn't either (and has an example with pass).

I guess I should raise an exception here, thank you.