How to alias `Annotated` to a subscriptable type

Empieza leyendo PEP 593 y los ejemplos de este issue; después, revisa cómo el typing project representa Annotated, los alias genéricos y class_getitem. El issue no especifica ningún archivo, prueba ni punto de entrada de la implementación, y para completarlo habría que determinar si este comportamiento debe admitirse y cómo debería especificarse.

Hi, I want to create a type alias that simply wraps a tuple into Annotated.

PEP 593

I want to be able to write

SpecialReturn[int, bool, str]

And get

Annotated[Tuple[int, bool, str], 'special return!']

Among other things, I've tried:

class SpecialReturn():
    def __class_getitem__(self, params: Tuple[Any, ...]) -> Tuple[Any, ...]:
        return typing.cast(
            Tuple[Any, ...],
            _typing.Annotated[
                typing.Tuple.__getitem__(params),
                'multiple_return',
            ]
        )

Which mypy is happy with the declaration, but not using it:

import typing

from typing import Annotated, Any, Tuple


class SpecialReturn():
    def __class_getitem__(self, params: Tuple[Any, ...]) -> Tuple[Any, ...]:
        return typing.cast(
            Tuple[Any, ...],
            Annotated[
                typing.Tuple.__getitem__(params),
                'multiple_return',
            ]
        )


def test() -> SpecialReturn[int, int]:
    return 1, 2

test8.py:17: error: "SpecialReturn" expects no type arguments, but 2 given
test8.py:18: error: Incompatible return value type (got "Tuple[int, int]", expected "SpecialReturn")

Which happens because I guess mypy does not evaluate the type annotation, it just treats SpecialReturn as a separate type. I am not sure what is the reasoning behind this, but I assume there it makes sense in some way(?).

So, is there any alternative? How do achieve this? I essentially want to alias Tuple and be able to find out at runtime if the type is a normal tuple or my special one.

My first jab at this, and what I think makes most sense to a Python developer is the following

from typing import Tuple


class SpecialReturn(Tuple):
    pass


def test() -> SpecialReturn[int, int]:
    return 1, 2

But mypy can't handle it either:

test.py:4: error: Missing type parameters for generic type "Tuple"
test.py:8: error: "SpecialReturn" expects no type arguments, but 2 given
test.py:9: error: Incompatible return value type (got "Tuple[int, int]", expected "SpecialReturn")

I was thinking maybe something like this was possible with Annotated, but it seems not.