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isinstance can take typing.Union as second arg in 3.10 and newer #12137

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hauntsaninja opened this issue Jun 13, 2024 · 3 comments
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isinstance can take typing.Union as second arg in 3.10 and newer #12137

hauntsaninja opened this issue Jun 13, 2024 · 3 comments

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@hauntsaninja
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Currently it allows for types.UnionType but not typing.Union

if sys.version_info >= (3, 10):
    _ClassInfo: TypeAlias = type | types.UnionType | tuple[_ClassInfo, ...]
else:
    _ClassInfo: TypeAlias = type | tuple[_ClassInfo, ...]

def isinstance(obj: object, class_or_tuple: _ClassInfo, /) -> bool: ...

I think mypy currently assumes that whatever typeshed says for isinstance has no false positives
@JelleZijlstra
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I'm not sure we could express this in typeshed. Union[int, str] is not an instance of typing.Union.

@AlexWaygood
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(I don't object to revisiting this, but this is a duplicate of #7505)

@hauntsaninja
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hauntsaninja commented Jun 13, 2024 •
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Thanks, looks like most of the relevant discussion is on #7508. Sounds like most people are surprised that isinstance works at all with old Union / think that type checkers should special case this even more than they currently do

@hauntsaninja hauntsaninja closed this as not planned Won't fix, can't repro, duplicate, stale Jun 13, 2024
@hauntsaninja hauntsaninja mentioned this issue Jun 13, 2024
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3 participants
@JelleZijlstra @hauntsaninja @AlexWaygood