Bug: __class_getitem__ not recognised

[samsja]

Bug Report

I am experienced a very weird behavior (which I am now pretty sure is a bug) with __class_getitem__.

To Reproduce

from abc import abstractmethod
from typing import Iterable


class A(Iterable):
    @abstractmethod
    def __class_getitem__(cls, item):
        ...


class B(A):
    def f(self, field: str):

        self.__class__[str]

>>> attribute.py:14: error: Value of type "Type[B]" is not indexable  [index]

Here I expect that mypy know that __class__ is bounded by the class B and therefore implement __class_getitem__.

The weirdest part is that removing the parameter in f remove the problem

from abc import abstractmethod
from typing import Iterable


class A(Iterable):
    @abstractmethod
    def __class_getitem__(cls, item):
        ...


class B(A):
    def f(self):

        self.__class__[str]

>>> Success: no issues found in 1 source file

Expected Behavior

mypy recognize that class can be indexed

Actual Behavior

mypy said it is no indexable even though B implement __class_getitem__

Your Environment

• Mypy version used: 0.990

• Mypy command-line flags: None

• Mypy configuration options from mypy.ini (and other config files): plugins = "pydantic.mypy"

• Python version used: 3.9

FYI: a more advance setup in this PR docarray/docarray#803

[tmke8]

Duplicate of #11501 I believe.

[AlexWaygood]

Duplicate of #11501 I believe.

Yup. Thanks!

[samsja]

I am not sure it is totally a duplicate. Did you see that if I don't have a parameters in my function mypy does not complain anymore ?
This work

    def f(self):
        self.__class__[str]

This does not

    def f(self, field: str):
        self.__class__[str]

Why is the behavior changing ?

[AlexWaygood]

The fundamental issue — that __class_getitem__ is an implementation detail of the typing system at runtime, which mypy has no special recognition of at present — is the same.

[AlexWaygood]

Did you see that if I don't have a parameters in my function mypy does not complain anymore ?

Oh, that's because your "function without parameters" is unannotated, meaning mypy just ignores it. It's a common footgun :)

If you want mypy to check that function, add -> None to the signature. Once you've done that, mypy emits an error for the version without parameters as well.

[samsja]

Oh I see ! Then yes it is a duplicate. Thanks for the clarification