false positive "assigning-non-slot" error for inherited descriptor when slots are used

[jab]

Bug description

Minimized reproducer:

"""Repro false positive bug in pylint."""


# pylint: disable=too-few-public-methods


class MyDescriptor:
    """Basic descriptor."""

    def __get__(self, instance, owner):
        return 42

    def __set__(self, instance, value):
        pass


class Base:
    """Base class with slots."""

    __slots__ = ()

    attr2 = MyDescriptor()


class Repro(Base):
    """Reproduce false positive error using this class."""

    __slots__ = ()


repro = Repro()
repro.attr2 = 'false positive "assigning-non-slot" error'  # false positive error here

A real-world (non-minimized) example where pylint produces this false positive error can be found on this line: https://github.com/jab/bidict/blob/caf703e959ed4471bc391a7794411864c1d6ab9d/bidict/_orderedbase.py#L101

Configuration

No response

Command used

pylint repro.py

Pylint output

************* Module repro
repro.py:32:0: E0237: Assigning to attribute 'attr2' not defined in class slots (assigning-non-slot)

Expected behavior

No error should be flagged here.

Pylint version

pylint 2.13.2
astroid 2.11.2
Python 3.10.2 (main, Feb  2 2022, 07:36:01) [Clang 12.0.0 (clang-1200.0.32.29)]

OS / Environment

No response

Additional dependencies

No response

[mbyrnepr2]

Regarding the provided example pasted above, I think this is expected Pylint behaviour due to attr2 not being one of the slots.

__slots__ = ("attr2",) vs. __slots__ = ()

[mbyrnepr2]

The answer here and here may be useful to achieve what you are looking for.

[jab]

Slots are for instance attributes, not class attributes. In this example, attr2 is (and should be) a class attribute, therefore it is omitted from slots, as it should be. Pylint is incorrectly treating it as an instance attribute, not the (inherited) class attribute that it actually is.

[mbyrnepr2]

Thanks for the info @jab. I certainly don't disagree with your analysis. The correct behaviour is also seen with the simpler case:

class Example:
    __slots__ = ('a',)


myinstance = Example()

myinstance.b = 1 # a.py:7:0: E0237: Assigning to attribute 'b' not defined in class slots (assigning-non-slot)

I think it does look like a false positive in your case :D Perhaps this line can be used to fix this also: https://github.com/PyCQA/pylint/blob/main/pylint/checkers/classes/class_checker.py#L1556

[jab]

@mbyrnepr2, your simplified example is not the same as what's going on here. In your example, you are assigning an instance attribute b. Since b is not present in slots for Example (or any of its base classes), pylint is correctly flagging the assigning-non-slot error.

This issue is about pylint incorrectly flagging an assigning-non-slot error when an inherited descriptor is used. (And note, the descriptor is a class attribute, so it should not be present in slots.) If you try to simplify the example I gave in the description to a case where a non-inherited descriptor is used, you can see that pylint correctly does not flag any assigning-non-slot error:

class MyDescriptor:
    def __get__(self, instance, owner):
        return 42
    def __set__(self, instance, value):
        instance.a = 42


class Example:
    __slots__ = ('a',)

    b = MyDescriptor()


myinstance = Example()
myinstance.b = 'set a to 42'  # no error on this line, as we expect
print(myinstance.a)  # prints 42

[mbyrnepr2]

@jab I agree with you. It's an example of correct usage vs. what you are reporting. Sorry that I didn't communicate that clearly.

[Hubrrr88]

I have seen the same issue on latest PyLint version 2.13.8. Any news on when will this be fixed yet?

[clavedeluna]

It appears this case was considered and tested here and deemed to need to emit this message. But I believe this is incorrect, as slots is meant for instance attributes, not class attributes.