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Mypy should allow for Self in parametrized parent class definition — issues with contextlib.AbstractContextManager #17758
Description
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 Anyand mypy disallows writing
class B(contextlib.AbstractContextManager[Self]): # error: Self type is only allowed in annotations within class definition [misc]
passand doesn't issue a warning for this runtime error:
class C(contextlib.AbstractContextManager[C]):
passFull 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 selfYour Environment
- Mypy version used: 1.11.2
- Mypy command-line flags: n/a
- Python version used: 3.11
Activity
Jazzinghen commentedon Jun 22, 2025
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:brianschubert commentedon Jun 22, 2025
Linking python/typeshed#13950 for related discussion
Jazzinghen commentedon Jun 23, 2025
I haven't read too much in the discussion, however it looks like they'd like to add a default value to
_EnterT_coof the abstract class definition.What I was wondering was if it was possible to actually use
Selfto 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 returnSelfinstead of having to write the type.sterliakov commentedon Sep 2, 2025
This is explicitly listed in the spec as disallowed: https://typing.python.org/en/latest/spec/generics.html#valid-locations-for-self
I don't think this is a sufficiently common problem to warrant a mypy-specific extension, so probably we should wait until semantics of Self parameterising inherited classes are specified.