`AbstractContextManger.__enter__` should be abstract

[KotlinIsland]

Code sample in basedpyright playground

from contextlib import AbstractContextManager
from typing import override


class CM(AbstractContextManager[int]):
    @override
    def __exit__(self, *_): ...

with CM() as i:
    print(i + 1)

AbstractContextManger.__enter__ has a default implementation that returns self, so by making it abstract you would require implementations to provide the correct implementation. this would force a redundant implementation if the type is Self, but that is currently impossible to type anyway: https://discuss.python.org/t/self-as-typevar-default/90939

even if we did have Self support, it would only require one line of redundant code: def __enter__(self) -> Self: return self

[srittau]

Honestly, I don't think the churn (and deviating from the implementation) is worth the slight advantage of "forcing" an implementation.

[KotlinIsland]

Honestly, I don't think the churn (and deviating from the implementation) is worth the slight advantage of "forcing" an implementation.

to me the purpose of an abstract class/protocol is to enforce that implementations are to meet the requirement, so to silently allow implementations to not match 50% of the contract is a mistake

i can understand that the class was written this because it's a reasonable default behaviour to return Self, but we don't have to technology to encode this into the type definition. it's not even currently possible for an implementation to opt-in to this default behaviour, which leads to issues like ExitStack

the churn

the primer for this change only picked up a single instance of an implementation depending on the default __enter__, I haven't looked into it, but it's possible it was just a missing from their stub file

[KotlinIsland]

@srittau i'm happy to handle any user reports caused by this change