How to use __class_getitem__?

[A-kirami]

I'm trying to use it like the following.

from typing import TypeVar

T = TypeVar("T")


class Ref:
    def __class_getitem__(cls, item: T) -> T:
        return item


a: Ref[int] = 0

But there was an error reported by pylance:

Expression of type "Literal[0]" cannot be assigned to declared type "Ref"
  "Literal[0]" is incompatible with "Ref"

I want it to be like typing. Annotated, how should I modify it?

[erictraut]

First, you're using __class_getitem__ in a manner for which it wasn't designed. Please refer to this documentation which specifically warns against this.

Second, you're using the expression Ref[int] in a type annotation for a variable a. The annotation Ref[int] is going to be interpreted by a type checker as "an instance of Ref parameterized with a type argument int. That means a must be assigned an instance of Ref, but you're attempting to assign it an instance of int, which a type checker will prevent.

Could you provide more information about what you're trying to do? If you want Ref to be generic, you should use the standard Generic base class, like this:

class Ref(Generic[T]): ...