(🐞) `isinstance` works with `typing.Union`s on 3.10+

[KotlinIsland]

from typing import Union

print(isinstance(1, Union[str, int]))  # error: Argument 2 to "isinstance" has incompatible type "object"; expected "Union[type, UnionType, Tuple[Union[type, UnionType, Tuple[Any, ...]], ...]]"

But this outputs True

Union[X, Y] is stubbed to return object when it really returns a typing._UnionGenericAlias to typing.Union so that would need to be fixed first.

playground

[srittau]

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: ...

I believe that his is purely a mypy bug, but someone could correct me here.

[KotlinIsland]

@srittau Thanks for the response!

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

Could you please elaborate?

[AlexWaygood]

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.

[KotlinIsland]

@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.

[AlexWaygood]

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!

[KotlinIsland]

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: ...

[KotlinIsland]

@AlexWaygood Here you go 😁 #7508

[AlexWaygood]

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".