BaseExceptionGroup should return ExceptionGroup if initialized with non-base exceptions #12972

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1. brianschubert commented on Nov 8, 2024

   

   brianschubert

   on Nov 8, 2024

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   Somewhat related: #9922

   At a glance it seems like an overloaded __new__ might work, similar to what's done with some of the other BaseExceptionGroup methods. Though given how special __new__ is, I suspect there may be some unique complications

   (cc @sobolevn as the author of the current BaseExceptionGroup overloads)
2. jakkdl commented on Jan 28, 2025

   

   jakkdl

   on Jan 28, 2025

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   Oh, found a comment in the test file noticing it as a limitation
   

   typeshed/stdlib/@tests/test_cases/builtins/check_exception_group-py311.py

   Lines 24 to 26 in aa992b9

   	# FIXME: this is not right, runtime returns `ExceptionGroup` instance instead,
   	# but I am unable to represent this with types right now.
   	assert_type(beg2, BaseExceptionGroup[ValueError])

   
   and found the accompanying discussion #9230 (comment) where @sobolevn lays out the tradeoffs

   I tried it out myself, and managed to repro the limitations*. Though I personally feel like getting BaseExceptionGroup -> ExceptionGroup is perhaps more important than custom BaseExceptionGroup subclasses; especially as end users can work around typing issues from custom BaseExceptionGroup with explicit type hints and/or adding a __new__ to their subclass.

   Though I also hit python/mypy#17251 so we'd need to remove the __init__, and I'm not sure what the downsides of that would be.

   * It's only a limitation for mypy, pyright handles this (somewhat minified) repro perfectly. Mypy falls back to defaults and raises some errors on the stub itself.

   from __future__ import annotations
   from typing import Generic, TypeVar, overload, Self
   from typing_extensions import reveal_type
   
   _BaseExceptionT_co = TypeVar("_BaseExceptionT_co", bound=BaseException, covariant=True, default=BaseException)
   _ExceptionT_co = TypeVar("_ExceptionT_co", bound=Exception, covariant=True, default=Exception)
   
   class BaseExceptionGroup(Generic[_BaseExceptionT_co]):
       @overload
       # mypy: Self argument missing for a non-static method (or an invalid type for self)
       def __new__(  # type: ignore[misc]
           cls: ExceptionGroup[_ExceptionT_co], _exception: _ExceptionT_co, /
       ) -> ExceptionGroup[_ExceptionT_co]: ...
       @overload
       def __new__(cls, _exception: _BaseExceptionT_co, /) -> Self: ...
   
       # mypy: "__new__" must return a class instance
       def __new__(  # type: ignore[misc]
           cls, _exception: _ExceptionT_co | _BaseExceptionT_co
       ) -> Self | ExceptionGroup[_ExceptionT_co]:
           return object.__new__(cls)
   
   class ExceptionGroup(BaseExceptionGroup[_ExceptionT_co]):
       def __new__(cls, _exception: _ExceptionT_co, /) -> Self:
           return object.__new__(cls)
   
   class MyBaseExcGroup(BaseExceptionGroup[_BaseExceptionT_co]): ...
   
   class MyExcGroup(ExceptionGroup[_ExceptionT_co]): ...
   
   reveal_type(BaseExceptionGroup(ValueError()))
   reveal_type(BaseExceptionGroup(SystemExit()))
   reveal_type(MyBaseExcGroup(ValueError()))  # mypy reverts to BaseException default
   reveal_type(MyBaseExcGroup(SystemExit()))
   reveal_type(ExceptionGroup(ValueError()))
   reveal_type(MyExcGroup(ValueError()))
   
   # expected errors
   ExceptionGroup(SystemExit())  # type: ignore[type-var]  # pyright: ignore[reportArgumentType]
   MyExcGroup(SystemExit())  # type: ignore[type-var]  # pyright: ignore[reportArgumentType]
3. srittau commented on Feb 26, 2025

   

   srittau

   on Feb 26, 2025

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   If someone could provide an exploratory PR, we could see the impact of such a change.
4. srittau

   added

   stubs: false positiveType checkers report false errorsType checkers report false errors

   on Feb 26, 2025