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pwuertz opened this issue Dec 4, 2023 · 3 comments
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Support for Union[*TypeVarTuple] #1523

pwuertz opened this issue Dec 4, 2023 · 3 comments
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topic: feature Discussions about new features for Python's type annotations

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@pwuertz
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pwuertz commented Dec 4, 2023

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] error

Mypy 1.7.1, Python 3.11
@pwuertz pwuertz added the topic: feature Discussions about new features for Python's type annotations label Dec 4, 2023
@erictraut
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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.

@pwuertz pwuertz closed this as completed Dec 31, 2023
@pwuertz pwuertz mentioned this issue Dec 31, 2023
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@matt-gardner
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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
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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 of Union[*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.

@pirate pirate mentioned this issue Oct 28, 2024
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@pwuertz @matt-gardner @erictraut