`abc.ABC.__slots__` should be marked with `ClassVar`

[embecka]

According to the python official documentation, slots are class variables. Therefore, in the class abc.ABC, the variable __slots__ should be annotated as ClassVar[Sequence[str]]. There is currently no annotation. It means that in all subclasses of abc.ABC, the variable __slots__ is considered as an instance variable.

[srittau]

__slots__ are usually not annotated at all. Instead they are initialized and should therefore be considered class vars by type checkers:

typeshed/stdlib/abc.pyi

Lines 45 to 46 in 11ff7e1

	class ABC(metaclass=ABCMeta):
	__slots__ = ()

In fact, I believe we should fix the few instances where we annotate __slots__.

[embecka]

The question is whether __slots__ can be annotated but does not have to be, or whether it can't be annotated.
Here is an example what I mean: mypy Playground
Regarding the class A, mypy does not complain that __slots__ is not annotated even in the strict mode.
But as we can see in the case of the class B, it assumes by default that __slots__ is an instance variable.
Now I am not sure if this is a bug of mypy, or it should be explicitly annotated in the stub file of abc.

[srittau]

I believe that mypy should have special handling for __slots__ as __slots__ is treated specially by Python. I would consider this a missing mypy feature. I don't there is much we can/want do about that in typeshed.

[srittau]

Cc @hauntsaninja as mypy and typeshed maintainer.

[JelleZijlstra]

Can you give a concrete example of code that is currently misinterpreted by a type checker and would be interpreted better with your proposed change?