Cannot parameterise typing_extensions.Protocol with a ParamSpec or TypeVarTuple on py310+

[AlexWaygood]

This change seems to have broken the latest version of pydash when used with typing-extensions 4.6.0. There's a bug report about this here: dgilland/pydash#197.

I did a bisect of every change between typing-extensions 4.5.0 and 4.6.0, and this was pointed out as the culprit. I've had a look at the code changes here but don't have a good enough grasp of what's going wrong that is causing this to fail.

This seems to be the relevant Pydash code that relates to the error:
https://github.com/dgilland/pydash/blob/051fe69c3e523f903a0a0ea6ca6a5c2d4b83a3e7/src/pydash/utilities.py#L577-L638

Originally posted by @Tenzer in #137 (comment)

[AlexWaygood]

My guess is that this change might be the culprit, but haven't checked that at all: https://github.com/python/typing_extensions/pull/137/files#r1164017024

[AlexWaygood]

Minimal repro:

>>> from typing_extensions import Protocol, ParamSpec
>>> P = ParamSpec("P")
>>> class Foo(Protocol[P]): ...
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\AppData\Local\Programs\Python\Python311\Lib\typing.py", line 344, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\typing_extensions\src\typing_extensions.py", line 672, in __class_getitem__
    raise TypeError(
TypeError: Parameters to Protocol[...] must all be type variables. Parameter 1 is ~P

Works fine on typing-extensions==4.5.0 (with Python 3.7 and Python 3.10), and also works fine if you use typing.{Protocol,ParamSpec}. So, definitely looks like a typing_extensions bug.

[AlexWaygood]

Interestingly, the bug only reproduces if you use Python >=3.10 (i.e., it has to be a version of Python where typing.ParamSpec is a thing for the bug to be reproducible).

[AlexWaygood]

Seems like the fundamental issue is that:

• We reimplement the logic from typing.Generic.__class_getitem__ in typing_extensions.Protocol.__class_getitem__. (Ideally we wouldn't have to do this, but the typing module on Python 3.7 doesn't let us have typing_extensions.Protocol subclass typing.Generic.)
• The implementation of typing.Generic.__class_getitem__ changed on Python 3.10 and Python 3.11, but we haven't reflected those changes in typing_extensions.
• Prior to typing_extensions==4.6.0, we only re-implemented Protocol on Python <=3.7, but now we re-implement Protocol on <=3.11. This means that the latent bug has been exposed on Python 3.10 and 3.11, hence the breakage.

[AlexWaygood]

Not that anybody asked, but the bug also means that you can't parameterise a typing_extensions.Protocol with a TypeVarTuple on 3.10/3.11:

>>> from typing_extensions import *
>>> Ts = TypeVarTuple("Ts")
>>> class Foo(Protocol[*Ts]): ...
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\AppData\Local\Programs\Python\Python311\Lib\typing.py", line 344, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\typing_extensions\src\typing_extensions.py", line 672, in __class_getitem__
    raise TypeError(
TypeError: Parameters to Protocol[...] must all be type variables. Parameter 1 is *Ts

[JelleZijlstra]

Thanks @AlexWaygood for investigating! Are you working on a fix?

[AlexWaygood]

I am looking into several potential fixes...

As usual with typing_extensions, there are several complications to grapple with :)

I also have a busy day at work, so might not be able to look at this properly until this evening!

[JelleZijlstra]

No hurry, I can give it a try later today too.

We'll probably want to release a bugfix 4.6.1 with this fix.

[AlexWaygood]

One possible fix involves copying over the logic for Generic.__class_getitem__ from 3.12a7 to typing_extensions.Protocol.__class_getitem__. In order to do that cleanly, though, we might have to first backport some unrelated fixes that were made to Unpack in 3.11, which comes with its own risks.

I looked into whether we could instead inherit from typing.Generic on 3.8+, which would mean we wouldn't have to have a Protocol.__class_getitem__ on 3.8+. I'm still investigating that, but suffice it to say that there are complications to that approach as well.

[srittau]

Just leaving this here for posterity:

[JelleZijlstra]

They're probably mostly referring to #179, not this issue.

[srittau]

Possible, #158 would probably have been a more fitting issue for the screenshot. But I claim innocence in any case. :)

[AlexWaygood]

Oh, I'm definitely guilty for this bug, if we're pointing fingers 😅

I agree with @JelleZijlstra though, there seems to be far more breakage from pydantic/pydantic#5821 than from this issue!

[AlexWaygood]

I am looking into several potential fixes...

As usual with typing_extensions, there are several complications to grapple with :)

I also have a busy day at work, so might not be able to look at this properly until this evening!

I have a fix in the works now.