Mypy should allow for Self in parametrized parent class definition — issues with contextlib.AbstractContextManager

[Hnasar]

Bug Report

AbstractContextManager defines a __enter__ that returns self

• https://github.com/python/typeshed/blob/6a8d653a671925b0a3af61729ff8cf3f90c9c662/stdlib/contextlib.pyi#L42
• https://github.com/python/cpython/blob/5436d8b9c397c48d9b0d5f9d4ad5e1d5a5d500f6/Lib/contextlib.py#L25

But it's impossible to just inherit from contextlib.AbstractContextManager and have the type of __enter__ be correctly assumed to be Self, without a bit of boilerplate.

To Reproduce

class A(contextlib.AbstractContextManager):
    pass

reveal_type(A().__enter__())  # should be Self but assumed to be Any

and mypy disallows writing

class B(contextlib.AbstractContextManager[Self]):  # error: Self type is only allowed in annotations within class definition  [misc]
    pass

and doesn't issue a warning for this runtime error:

class C(contextlib.AbstractContextManager[C]):
    pass

Full example here
https://mypy-play.net/?mypy=latest&python=3.11&gist=7e76e56be6c702005a7ffbb7846b69c2

Expected Behavior

Either Self is allowed in the AbstractContextManager parameter list, or the type vars work by default.

Actual Behavior

Errors. See the playground.
(Although pyright doesn't error when using Self in the parameter list, but it still wrongly infers Any as the return of __enter__)

Workaround

Repeat the class name, and repeat the __enter__ impelmentation

class E(contextlib.AbstractContextManager["E"]):
    def __enter__(self) -> Self:
        return self

Your Environment

• Mypy version used: 1.11.2
• Mypy command-line flags: n/a
• Python version used: 3.11

[Jazzinghen]

I encountered this issue today as well.
I was expecting this to work, especially when considering the following lines in the documentation of typing.Self:

Other common use cases include:

• classmethods that are used as alternative constructors and return instances of the cls parameter.
• Annotating an __enter__() method which returns self.

[brianschubert]

Linking python/typeshed#13950 for related discussion

[Jazzinghen]

I haven't read too much in the discussion, however it looks like they'd like to add a default value to _EnterT_co of the abstract class definition.

What I was wondering was if it was possible to actually use Self to write something like @Hnasar wrote. In that way we can maintain the genericity of the Protocol definition while also not become crazy in writing the types ourselves in case we want to make a base context manager class that expects all the classes inheriting from it to return Self instead of having to write the type.