8000 `classmethod`s shouldn't be allowed to access generic attributes · Issue #11537 · python/mypy · GitHub
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KotlinIsland opened this issue Nov 13, 2021 · 2 comments
Open

classmethods shouldn't be allowed to access generic attributes #11537

KotlinIsland opened this issue Nov 13, 2021 · 2 comments
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bug mypy got something wrong

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@KotlinIsland
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KotlinIsland commented Nov 13, 2021 •
edited
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from typing import Generic, TypeVar

T = TypeVar("T")


class A(Generic[T]):
    class_attr: T

    @classmethod
    def set_attr(cls, arg: T) -> None:
        cls.class_attr = arg  # no error?


B = A[str]
B.set_attr("1")
B.class_attr = "1"  # error: Access to generic instance variables via class is ambiguous

C = A[int]
C.set_attr(1)
C.class_attr = 1  # error: Access to generic instance variables via class is ambiguous

value = B().class_attr

reveal_type(value)  # revealed: str, actual value: int
@KotlinIsland KotlinIsland added the bug mypy got something wrong label Nov 13, 2021
@KotlinIsland KotlinIsland changed the title classmethods shouldn't be allowed to have generics in their signatures classmethods shouldn't be allowed to access generic attributes Nov 13, 2021
@DetachHead
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related: #9456 (comment)

@Dreamsorcerer
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As per #9456 (comment), the error here is that mypy allows a TypeVar for a class attribute (which should be annotated with ClassVar, unlike in the example), and also allows assigning to an instance attribute in a classmethod.

@DetachHead DetachHead mentioned this issue Nov 13, 2021
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davidhalter added a commit to davidhalter/mypy that referenced this issue Feb 20, 2023
@davidhalter
Probably a mypy bug related to python#11537
6d367c3
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@Dreamsorcerer @DetachHead @KotlinIsland
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