PEP 646 implementation

[JelleZijlstra]

Fixespython/typing_extensions#28.

This turned out to require even more hacks than usual, because the runtime in old versions of typing really doesn't want things that are not TypeVars to be used as TypeVars.

The tests were copied from https://github.com/python/cpython/pull/24527/files and the implementation is mostly inspired by Required/NotRequired (Unpack) and ParamSpec (TypeVarTuple).

[JelleZijlstra]

The last 3.6 failures are annoying and I don't see an easy fix, I think I'll give it another try tomorrow.

[JelleZijlstra]

This is now finally ready for review. This is uncommonly hacky even by typing-extensions standards, because the older versions of typing only know about TypeVars and now we need to convince them to somehow also accept unpacked TypeVarTuples. I solved this with monkeypatching (yuck).

For 3.6, which had a very different internal implementation of Generic, it's still not enough. Given that 3.6 is EOL and most usage of TypeVarTuple/Unpack actually works correctly, I decided to just leave this as a documented limitation.

[Fidget-Spinner]

@JelleZijlstra Sorry I'll try to review this in the next week or two, swamped right now. Ping me again if I forget please!

[JelleZijlstra]

No hurry, take your time!

[Fidget-Spinner]

Looks good in general. Thanks Jelle! BTW, should I be reviewing this or Mathew's CPython PR first? I'd imagine any tweaks to his PR would affect yours.

[Fidget-Spinner]

I think Mathew pointed out in his documenting typing.py PR that parameters is confusing. Should we use args or type_args instead?

Also what's elen's default for? When would we not want to pass in __parameters__ ourselves?

[JelleZijlstra]

This is a modified copy of typing.py's _check_generic, and it's monkeypatched into typing later. We probably shouldn't modify the function's interface, because that makes the monkeypatching even riskier.

[Fidget-Spinner]

To clarify, this means even with our monkey patching, subscripting with arbitrary number of type arguments won't work right?

[JelleZijlstra]

Defining the class doesn't even work. I'll make the note more precise.

Python 3.6.10 (default, Apr 28 2021, 18:14:25) [GCC Apple LLVM 12.0.0 (clang-1200.0.32.29)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import Generic
>>> from typing_extensions import Unpack, TypeVarTuple
>>> Ts = TypeVarTuple("Ts")
>>> class X(Generic[Unpack[Ts]]): pass
... Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/jelle/.pyenv/versions/3.6.10/lib/python3.6/typing.py", line 965, in __new__
", ".join(str(g) for g in gvars)))
TypeError: Some type variables (Ts) are not listed in Generic[typing_extensions.Unpack[Ts]]

[JelleZijlstra]

Thanks! I think it's better to focus on this first, since 3.11 has months of time left in any case, but with typing-extensions we can release at any time.

However, I'll look at his current PR for things to improve here; I think he wrote better docstrings.

[srittau]

I won't claim to really understand what is going on with typing internals, but I see no red flags.

[mrahtz]

Thanks all - especially @JelleZijlstra!