Unpacking TypeAliasType to tuple of types using * errors at runtime, Unpack doesn't, pyright passes, bug in cpython or pyright?

[alwaysmpe]

I'm writing classes using variadic generic code and as a minimal example I have the below code which raises no type errors on pyright playground and is crashing at runtime which I believe is a bug in cpython, would like some advice if possible. The code:

from __future__ import annotations
from typing import Unpack, assert_type
type FooArgT = tuple[str, str]

class Foo[*Arg]:
    pass

type FooT = Foo[str, *FooArgT]
type FooUT = Foo[str, Unpack[FooArgT]]


class BarU(Foo[str, Unpack[FooArgT]]):
    pass

def foo_fn(t_arg: FooT, ut_arg: FooUT):
    assert_type(t_arg, FooT)
    assert_type(t_arg, FooUT)
    assert_type(t_arg, Foo[str, Unpack[FooArgT]])
    assert_type(t_arg, Foo[str, *FooArgT])
    assert_type(ut_arg, FooT)
    assert_type(ut_arg, FooUT)
    assert_type(ut_arg, Foo[str, Unpack[FooArgT]])
    assert_type(ut_arg, Foo[str, *FooArgT])

class Bar(Foo[str, *FooArgT]):
    pass

When I run this locally, I get:

Traceback (most recent call last):
  File "...", line 25, in <module>
    class Bar(Foo[str, *FooArgT]):
                  ^^^^^^^^^^^^^
TypeError: Value after * must be an iterable, not typing.TypeAliasType

By my understanding, the 2 class declarations are equivalent, pyright checks pass and cpython should allow unpacking of the aliased TypeVarTuple. A cursory look at the cpython issues I can't see anything equivalent so figured here might be a good place to start.

I'm running python3.13.0 in wsl installed using pyenv, output of python -V -V

Python 3.13.0 (main, Oct 24 2024, 00:12:39) [GCC 11.4.0]

Or should I just pack in this whole python thing and go back to c++?

[erictraut]

@JelleZijlstra, what do you think? Should TypeAliasType work with an unpack operator? If not, it probably makes sense to add a check to static type checkers that reports an error if an unpack operator is applied to a TypeAliasType outside of a type annotation.

[JelleZijlstra]

I think we should make this work at runtime. @alwaysmpe could you open an issue on https://github.com/python/cpython about this?

[alwaysmpe]

Will do!

[alwaysmpe]

python/cpython#126085 created!