(🐞) isinstance works with typing.Unions on 3.10+ #7505

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1. srittau commented on Mar 17, 2022

   

   srittau

   on Mar 17, 2022

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   The stubs look correct to me:

   typeshed/stdlib/builtins.pyi

   Lines 1161 to 1172 in e8286e7

   	# We need recursive types to express the type of the second argument to `isinstance` properly, hence the use of `Any`
   	if sys.version_info >= (3, 10):
   	def isinstance(
   	__obj: object, __class_or_tuple: type | types.UnionType | tuple[type | types.UnionType | tuple[Any, ...], ...]
   	) -> bool: ...
   	def issubclass(
   	__cls: type, __class_or_tuple: type | types.UnionType | tuple[type | types.UnionType | tuple[Any, ...], ...]
   	) -> bool: ...
   	else:
   	def isinstance(__obj: object, __class_or_tuple: type | tuple[type | tuple[Any, ...], ...]) -> bool: ...
   	def issubclass(__cls: type, __class_or_tuple: type | tuple[type | tuple[Any, ...], ...]) -> bool: ...

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   I believe that his is purely a mypy bug, but someone could correct me here.
2. KotlinIsland commented on Mar 17, 2022

   

   KotlinIsland

   on Mar 17, 2022

   Last edited by KotlinIsland

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   @srittau Thanks for the response!

   That stub doesn't contain anything about typing.Union.

   Could you please elaborate?
3. AlexWaygood commented on Mar 17, 2022

   

   AlexWaygood

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   Strictly speaking the Union[str, int] object being passed to the union is of type typing._SpecialForm (in the stubs at least — not at runtime) and it's true that the stub doesn't currently allow for the possibility of passing in _SpecialForm objects.

   I'm not sure this is really a problem that should be solved in the stubs, though. If we add _SpecialForm to the union of types that's allowed for the second argument, that might mean that type checkers might start allowing things like isinstance(1, Literal[1]), which fails at runtime. Given that it's really only this specific special form that works with isinstance, I think it's probably better for a type checker to special-case it.
4. KotlinIsland commented on Mar 17, 2022

   

   KotlinIsland

   on Mar 17, 2022

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   @AlexWaygood Actually it's more specifically a typing._UnionGenericAlias to typing.Union which is an instance of _SpecialForm . We could just stub a fake subtype of _SpecialForm only for Unions, and use that to annotate.

   I could make a pr when I get time.
5. AlexWaygood commented on Mar 17, 2022

   

   AlexWaygood

   on Mar 17, 2022

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   I could make a pr when I get time.

   Cool, I'll happily take a look at a PR!

   In general I'm slightly leery of overcomplicating typing.pyi by reproducing the exact runtime implementation details, but if there's a way to do it simply, then sure!
6. KotlinIsland commented on Mar 17, 2022

   

   KotlinIsland

   on Mar 17, 2022

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   Something roughly like this:

   class _SpecialForm:
       def __getitem__(self, typeargs: Any) -> object: ...
   
   class _UnionSpecialForm(_SpecialForm):
       def __getitem__(self, typeargs: Any) -> _UnionSpecialForm: ...
   
   Union: _UnionSpecialForm = ...

    def isinstance(__obj: object, __class_or_tuple: type | types.UnionType | _UnionSpecialForm) -> bool: ...

   👍React with 👍1Reacted by Alex Waygood
7. KotlinIsland commented on Mar 17, 2022

   

   KotlinIsland

   on Mar 17, 2022

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   @AlexWaygood Here you go 😁 #7508
8. AlexWaygood

   changed the title [-](🐞) `isinstance` works with `typing.Union`s[/-] [+](🐞) `isinstance` works with `typing.Union`s on 3.10+[/+] on Mar 18, 2022
9. AlexWaygood commented on Jun 12, 2022

   

   AlexWaygood

   on Jun 12, 2022

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   The consensus in #7508 was that this was an unusual and niche use case, so it's probably best to close this as a "wontfix".
10. AlexWaygood

    closed this as not plannedon Jun 12, 2022