Is it possible to define a subscriptable class which can take type arguments and ellipses?

[langfield]

• Are you reporting a bug, or opening a feature request?

More of a feature request/question.

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

The following is a mock up and not fully implemented. I would like to define a class for which the defined functions and the statements written in main() will not raise mypy errors with the config file I've given below. Or at least I would like to know if this is possible or not.

from typing import Any, TypeVar
from functools import lru_cache

T = TypeVar("T")


class SubscriptableType(type):
    """ Generic metaclass for subscriptable type. """

    def __init_subclass__(cls) -> None:
        cls._hash = 0

    @lru_cache()
    def __getitem__(cls, item: Any) -> type:
        body = {
            **cls.__dict__,
            "__args__": item,
            "__origin__": cls,
        }
        bases = cls, *cls.__bases__
        result = type(cls.__name__, bases, body)
        return result

    def __eq__(cls, other: Any) -> bool:
        cls_args = getattr(cls, "__args__", None)
        cls_origin = getattr(cls, "__origin__", None)
        other_args = getattr(other, "__args__", None)
        other_origin = getattr(other, "__origin__", None)
        args_eq: bool = cls_args == other_args
        origins_eq: bool = cls_origin == other_origin
        is_eq: bool = args_eq and origins_eq
        return is_eq

    def __hash__(cls) -> int:
        """ TODO: implement. """
        if not getattr(cls, "_hash", None):
            cls._hash = 0
        return cls._hash


class AMeta(SubscriptableType):
    pass


class AType(metaclass=AMeta):
    pass


def function_1(x: AType[int]) -> AType[int]:
    return x


def function_2(x: AType[...]) -> AType[...]:
    return x


def function_3(x: AType[None]) -> AType[None]:
    return x


def main() -> None:
    x = 0
    isinstance(x, AType[int])
    isinstance(x, AType[...])

• What is the actual behavior/output?

Given the following mypy config:

[mypy]
check_untyped_defs = true
disallow_untyped_defs = true
ignore_missing_imports = true
no_implicit_optional = true
warn_redundant_casts = true
warn_return_any = true
warn_unused_ignores = true
show_error_codes = true

# Ignore all torch type errors.
[mypy-torch.*]
follow_imports = skip
follow_imports_for_stubs = True

Mypy output:

asta/demo.py:49: error: "AType" expects no type arguments, but 1 given  [type-arg]
asta/demo.py:53: error: Unexpected '...'  [misc]
asta/demo.py:53: error: "AType" expects no type arguments, but 1 given  [type-arg]
asta/demo.py:57: error: "AType" expects no type arguments, but 1 given  [type-arg]

• What is the behavior/output you expect?
  I would like the above to raise no mypy errors for someone using the class AType in their own projects. I'm fine with disabling errors in the source code of the class/metaclass definitions, but I would like to be able to use AType in place of a subscriptable generic type in type hints, and raise no errors if passed None, ..., '(int, int), or intfor example. Inheriting fromGeneric[T]is out of the question because it will make mypy complain aboutisinstance()` checks.

• What are the versions of mypy and Python you are using?

Python 3.7.6
Mypy 0.761

Do you see the same issue after installing mypy from Git master?
I trying to determine if this is possible for the latest version available from pip.

• What are the mypy flags you are using? (For example --strict-optional)
  See above. Ran with mypy <directory>.

• If mypy crashed with a traceback, please paste
  the full traceback below.
  Did not crash.

(You can freely edit this text, please remove all the lines
you believe are unnecessary.)