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Support for Union[*TypeVarTuple] #1523
Description
I'm trying to annotate a method that only accepts a union of specific types, with the types specified via variadic generic. Although mypy supports variadic unpacking like Tuple[*Ts], unpacking Union[*Ts] doesn't seem to work yet:
import typing
Ts = typing.TypeVarTuple("Ts")
class Accepts(typing.Generic[*Ts]):
# error: Unpack is only valid in a variadic position [valid-type]
def foo(self, x: typing.Union[*Ts]): ...
class A: ...
class B: ...
class C: ...
class Test(Accepts[A, B]): ...
Test().foo(A()) # ok
Test().foo(B()) # ok
Test().foo(C()) # should raise [arg-type] errorMypy 1.7.1, Python 3.11
Reacted by Kazantcev Andrey, Javier Leguina and stan
Activity
erictraut commentedon Dec 22, 2023
This is simply a missing piece of functionality in mypy. I recommend closing this issue and opening a corresponding one in the mypy issue tracker to it gets the attention of the mypy maintainers.
FWIW, the above code type checks as expected in pyright.
matt-gardner commentedon Feb 1, 2024
Apologies if I'm missing something, but did pyright's behavior change since the above comment was written? When I click on the link to the pyright playground, I see a failure on line 7.
erictraut commentedon Feb 1, 2024
Yes, pyright's behavior changed since the above comment. Unbeknownst to me, the PEP 646 authors removed the
Union[*Ts]capability prior to the PEP's acceptance. I implemented pyright's PEP 646 support prior to this removal. Since the final version of the PEP removed the notion ofUnion[*Ts], I likewise removed support for it in pyright.If you would like to see
Union[*Ts]become part of the type spec, that would require a new PEP, or at least a public discussion in the Typing forum, a proposed modification to the spec, and approval by the Typing Council.