类型：如何将所有者类绑定到通用描述符？

问题描述

我可以用Python支持/尊重/理解其所有者的继承体系的方式来实现通用描述符吗？

代码中应该更清楚：

from typing import (
    Generic,Optional,TYPE_CHECKING,Type,TypeVar,Union,overload,)

T = TypeVar("T",bound="A")  # noqa

class Descr(Generic[T]):

    @overload
    def __get__(self: "Descr[T]",instance: None,owner: Type[T]) -> "Descr[T]": ...

    @overload
    def __get__(self: "Descr[T]",instance: T,owner: Type[T]) -> T: ...

    def __get__(self: "Descr[T]",instance: Optional[T],owner: Type[T]) -> Union["Descr[T]",T]:
        if instance is None:
            return self
        return instance


class A:
    attr: int = 123
    descr = Descr[T]()  # I want to bind T here,but don't kNow how


class B(A):
    new_attr: int = 123
    qwerty: str = "qwe"


if __name__ == "__main__":
    a = A()
    if TYPE_CHECKING:
        reveal_type(a.descr)  # mypy guess it is T? but I want A*
    print("a.attr =",a.descr.attr)  # mypy error: T? has no attribute "attr"
                                     # no runtime error
    b = B()
    if TYPE_CHECKING:
        reveal_type(b.descr)  # mypy said it's T? but I want B*

    print("b.new_attr =",b.descr.new_attr)  # mypy error: T? has no attribute "new_attr"
                                             # no runtime error
    print("b.qwerty =",b.descr.qwerty)  # mypy error: T? has no attribute "qwerty"
                                         # (no runtime error)

gist-要点上几乎相同的代码段

解决方法

我不确定您是否需要将描述符类设为通用类；将__get__的实例上的Type[T]返回T可能就足够了：

T = TypeVar("T")  # noqa

class Descr:
    @overload
    def __get__(self,instance: None,owner: Type[T]) -> "Descr": ...

    @overload
    def __get__(self,instance: T,owner: Type[T]) -> T: ...

    def __get__(self,instance: Optional[T],owner: Type[T]) -> Union["Descr",T]:
        if instance is None:
            return self
        return instance


class A:
    attr: int = 123
    descr = Descr()


class B(A):
    new_attr: int = 123
    qwerty: str = "qwe"

您的每个示例都能按您的意愿工作，并且将会出现错误

print("b.spam =",b.descr.spam)

产生

error: "B" has no attribute "spam"