Fix ParamSpec ellipsis default for <3.10

[Gobot1234]

No description provided.

[AlexWaygood]

Could you add a test?

[AlexWaygood]

CHANGELOG entry/docs? :-)

If this fixes an issue you hit, other people might have hit it too, so they probably deserve to know about the fix!

[JelleZijlstra]

The change itself looks good, but please add a changelog/docs entry as Alex requested. For reference, PEP 696 explicitly allows this: https://peps.python.org/pep-0696/#paramspec-defaults

[Gobot1234]

Oh weird, it doesn't seem to have pushed the changes

[JelleZijlstra]

Looks good but @AlexWaygood please take a look too.

I'll likely cut a new release once the PEP 727 (Doc()) changes are in.

[AlexWaygood]

Aha, so the issue is that typing._type_check changed in Python 3.11 so that it would accept an ellipsis, but the function didn't allow it on previous versions of CPython

[AlexWaygood]

Looks reasonable to me, other than my point about the docs.

Another possible fix could be to vendor the current version of _type_check from the CPython main branch, since we use that function quite a lot in typing_extensions, and there might have been other changes to _type_check in recent versions of CPython that are causing bugs we don't know about yet in typing_extensions. It would be a more invasive change, though; I think this fix is fine for now

[Gobot1234]

Looks reasonable to me, other than my point about the docs.

Another possible fix could be to vendor the current version of _type_check from the CPython main branch, since we use that function quite a lot in typing_extensions, and there might have been other changes to _type_check in recent versions of CPython that are causing bugs we don't know about yet in typing_extensions. It would be a more invasive change, though; I think this fix is fine for now

I think its this change FWIW python/cpython@870b22b

[AlexWaygood]

I think its this change FWIW python/cpython@870b22b

Riiight... so we probably can't use typing_extensions.Annotated to wrap typing_extensions.ParamSpecArgs on lower versions of Python... probably not the biggest problem in the world... :)

[AlexWaygood]

thanks, lgtm!

[AlexWaygood]

I think its this change FWIW python/cpython@870b22b

Riiight... so we probably can't use typing_extensions.Annotated to wrap typing_extensions.ParamSpecArgs on lower versions of Python... probably not the biggest problem in the world... :)

lol, confirmed:

Python 3.9.12 (main, Apr  4 2022, 05:22:27) [MSC v.1916 64 bit (AMD64)] :: Anaconda, Inc. on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing_extensions import *
>>> P = ParamSpec("P")
>>> Annotated[P.args, 'foo']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\.conda\envs\py39\lib\typing.py", line 277, in inner
    return func(*args, **kwds)
  File "C:\Users\alexw\.conda\envs\py39\lib\typing.py", line 1340, in __class_getitem__
    origin = _type_check(params[0], msg, allow_special_forms=True)
  File "C:\Users\alexw\.conda\envs\py39\lib\typing.py", line 166, in _type_check
    raise TypeError(f"{msg} Got {arg!r:.100}.")
TypeError: Annotated[t, ...]: t must be a type. Got P.args.

Works fine with typing on later Python versions:

Python 3.11.2 (tags/v3.11.2:878ead1, Feb  7 2023, 16:38:35) [MSC v.1934 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import *
>>> P = ParamSpec("P")
>>> Annotated[P.args, 'foo']
typing.Annotated[P.args, 'foo']

[JelleZijlstra]

We probably should vendor _type_check unconditionally. It's not a particularly likely thing to run into, but it is technically a bug in typing-extensions. Do you want to make a PR?

[AlexWaygood]

Sure

[AlexWaygood]

We probably should vendor _type_check unconditionally.

hmm... seems tricky to just copy over the py311+ implementation of _type_convert without also backporting _type_convert() from py311+ (_type_convert is called by _type_check()), and that in turn requires backporting ForwardRef as it exists on py311+ (ForwardRef is instantiated inside _type_convert(), and is instantiated using new arguments that were only added to ForwardRef.__init__ in py311).

[JelleZijlstra]

Thanks for investigating! That feels like it might be more trouble than a purely hypothetical use case is worth. Adding our own ForwardRef would potentially break users who do isinstance(..., typing.ForwardRef). (They should technically be checking for typing_extensions.ForwardRef anyway, but we don't have to break them unnecessarily.)