Add __class_getitem__ to support runtime type parameters for some classes

[Sobolev Nikita]

We (in django-stubs) treat several types as Generics. We provide type parameters to them like so: QuerySet[MyModel]

Currently it is impossible without a monkeypatch from our side, because these classes do not have __class_getitem__ method.
Suggested patch will address this problem for:

• cached_property
• Lookup
• Field
• QuerySet
• BaseManager

Implementation detail: __class_getitem__ will be a no-op for django runtime. Since types are only required during mypy check.

django-stubs: ​https://github.com/typeddjango/django-stubs
monkey-patching: ​https://github.com/typeddjango/django-stubs/blob/5b3088a17a76a77cb56a9c2241c2ddc6ccc8153a/mypy_django_plugin/django/context.py#L60-L61
Related DEP: ​https://github.com/django/deps/pull/65

[Ran Benita]

Instead of adding a custom dummy __class_getitem__() implementation, another approach is to actually inherit from typing.Generic. As long as Django doesn't publish its types with a py.typed file, it should not have an effect on type-checking, but it will add the necessary __class_getitem__ at runtime: ​https://github.com/python/cpython/blob/78c7183f470b60a39ac2dd0ad1a94d49d1e0b062/Lib/typing.py#L876

The reason to do this is that presumably the __class_getitem__ runtime implementation in typing is more correct (certainly seems to do a lot) and more forward compatible than a custom implementation in Django would be.

There might be repercussions I am not considering though, like performance, or side effects of importing typing.

[Sobolev Nikita]

Replying to Ran Benita:

Yes, this is a best-practice indeed. But.
Inheriting from Generic will also modify __init_subclass__ and __new__ methods. And I am not sure about it.
I guess for now just __class_getitem__ is enough. It has zero possible problems and works for us.

Later (when working on next typing-related things) we would need to return to this question once again.

[David Foster]

Based on one +0 and 2 +1s for this change at ​https://github.com/django/deps/pull/65 , I'm marking it as Accepted.

[David Foster]

​PR, by reporter

[Carlton Gibson <carlton@…>]

In 578c03b:

Fixed #31223 -- Added class_getitem() to Manager and QuerySet.

[GitHub <noreply@…>]

In 0de89b6:

Refs #31223 -- Added class_getitem() to ForeignKey.