Descriptors and ClassVars

Bug Report

Have came across this issue from pyright

To Reproduce

# This sample tests the case where a member access is performed through
# an object using a field that is annotated as a ClassVar. Normally this
# is disallowed, but it is permitted if the type of the ClassVar is
# a descriptor object.
from __future__ import annotations

from typing import TYPE_CHECKING, ClassVar, Generic, Self, TypeVar, overload

T = TypeVar("T")


class Descriptor(Generic[T]):
    if TYPE_CHECKING:

        @overload
        def __get__(self, instance: None, owner) -> Self:
            ...

        @overload
        def __get__(self, instance: object, owner) -> T:
            ...

        def __get__(self, instance: object | None, owner) -> Self | T:
            ...

        def __set__(self, instance: object, value: T) -> None:
            ...

        def is_null(self) -> bool:
            ...


class Example:
    field1: ClassVar = Descriptor[str]()

    field2: ClassVar = ""

    def reset(self) -> None:
        self.field1 = ""

        # This should generate an error because field2 isn't
        # a descriptor object.
        self.field2 = ""

Expected Behavior

Copied @erictraut comment:

ClassVar should work fine. If the class variable is assigned a descriptor, then the class variable will not be overwritten if you set the same attribute at the object level. So it sounds like mypy has a bug here. Pyright had a similar bug, which I recently fixed. Please report the bug to the maintainers of mypy.

Actual Behavior

classvar.py:39: error: Cannot assign to class variable "field1" via instance  [misc]
classvar.py:43: error: Cannot assign to class variable "field2" via instance  [misc]

Your Environment

• Mypy version used: 1.1.1
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.11.2

Commencez par exécuter le reproducteur Python fourni avec mypy 1.1.1 et comparez les diagnostics pour le field1 pris en charge par un descriptor et le field2 ClassVar ordinaire. Suivez ensuite le traitement des affectations de membres impliqué dans ces diagnostics, puis ajoutez une couverture de régression montrant que l’affectation prise en charge par un descriptor est acceptée, tandis que l’affectation ordinaire à ClassVar reste une erreur.