Add Y036: check for badly-defined __(a)exit__ methods

[AlexWaygood]

Fixes #188

[AlexWaygood]

There is one remaining typeshed hit, which I'm not quite sure what to do with:

https://github.com/python/typeshed/blob/ec27c00ca220c501a55e51617117f5b2d60fc06c/stdlib/imp.pyi#L48

The check I've written argues that it should be rewritten:

def __exit__(self, *args: Any) -> Any: ...  # OLD
def __exit__(self, *args: object) -> Any: ...  # NEW

The only thing is, this method is part of a protocol definition. I'm not sure whether it's important to keep it as it is or not. I think it would probably be fine? But also protocols generally need more permissive definitions for argument types. So, I'm not sure.

If we decide not to change that method in typeshed, we have two options:

1. # noqa it in typeshed.
2. Add further complexity to this check by special-casing __(a)exit__ methods inside protocol definitions.

[Akuli]

On the one hand, I agree with flake8 that this is quite complicated. On the other hand, it's good because all exit method checking is in the same place.

[AlexWaygood]

Yes. I tried splitting it up, and, in my opinion, it made the code much less readable.

[Akuli]

The typeshed hit seems buggy. Isn't it saying that for an object to be compatible with that protocol, it must have an exit method that accepts any number of arguments?

To test this, I made this toy example:

from types import TracebackType
from typing import Any
from typing_extensions import Protocol

class AbstractFoo(Protocol):
    def __exit__(self, *args: Any) -> None: ...

class ConcreteFoo(AbstractFoo):
    def __exit__(self, typ: type[BaseException] | None, exc: BaseException | None, tb: TracebackType | None) -> None:
        print("Hi")

f: AbstractFoo = ConcreteFoo()

Mypying it fails with:

asd.py:9: error: Signature of "__exit__" incompatible with supertype "AbstractFoo"
asd.py:9: note:      Superclass:
asd.py:9: note:          def __exit__(self, *Any) -> None
asd.py:9: note:      Subclass:
asd.py:9: note:          def __exit__(self, Optional[Type[BaseException]], Optional[BaseException], Optional[TracebackType]) -> None
Found 1 error in 1 file (checked 1 source file)

[Akuli]

I get that and it's fine, but *args in a Protocol seems to be broken and a bad idea.

[AlexWaygood]

The typeshed hit seems buggy. Isn't it saying that for an object to be compatible with that protocol, it must have an exit method that accepts any number of arguments?

Hah, good point. So the stub is broken anyway.

[AlexWaygood]

I get that and it's fine, but *args in a Protocol seems to be broken and a bad idea.

Yeah sorry, I misread your message :)

[Akuli]

One more nit.

[AlexWaygood]

Thanks for the reviews!