如何让mypy识别__init_subclass__中定义的ClassVar类型？

To resolve the mypy type checking warnings, you need to make the Philosopher class generic, tying its school class variable to the concrete School subclass passed during subclass creation. Here's the fixed code:

from typing import TypeVar, ClassVar, Generic, type

class School:
    def __init__(self) -> None:
        self.number = 0

    def test(self) -> None:
        self.number = 0


class Sophism(School):
    def test(self) -> None:
        self.number = 1


class Epicureanism(School):
    def test(self) -> None:
        self.number = 2


# Define a TypeVar bound to School, to represent concrete subclasses
SchoolT = TypeVar("SchoolT", bound=School)


class Philosopher(Generic[SchoolT]):
    # ClassVar uses the generic type variable to track the concrete School subclass
    school: ClassVar[SchoolT]

    def __init_subclass__(cls, /, school: type[SchoolT], **kwargs: object) -> None:
        super().__init_subclass__(**kwargs)
        cls.school = school()


# Explicitly specify the generic type when creating the subclass
class Sophist(Philosopher[Sophism], school=Sophism):
    pass


s1 = Sophist()  
s2 = Sophist()
s1.school.test()  # No mypy error now
assert s1.school.number == s2.school.number == Sophist.school.number  # True
assert s1.school == s2.school == Sophist.school  # True

Key Changes: #

• Generic Philosopher Class: By inheriting from Generic[SchoolT], we make Philosopher a generic class where each subclass is tied to a specific School type. This lets mypy infer the concrete type of school for each subclass.
• ClassVar with Generic Type: The school attribute is annotated as ClassVar[SchoolT], which tells mypy it's a class-level variable whose type matches the generic parameter of the subclass.
• Explicit Generic Parameter: When defining Sophist, we specify Philosopher[Sophism] to bind the subclass to the Sophism school type. This ensures mypy recognizes s1.school as an instance of Sophism, with access to test() and number.
• Type Hint for init_subclass: The school parameter in __init_subclass__ is typed as type[SchoolT], confirming it's a subclass of School that matches the generic parameter.

These changes eliminate all mypy warnings while preserving the original runtime behavior.

内容的提问来源于stack exchange，提问作者Some Guy