Signature incorrect for Coroutine throw?

[alicederyn]

I own some code that has been forwarding throw by declaring a three-parameter method with two defaulted to None, but as of py3.11 we have started seeing errors:

TypeError: throw() third argument must be a traceback

This doesn't seem to match the type stubs. Should the second Coroutine.throw overload be changed to remove the final | None?

typeshed/stdlib/typing.pyi

Line 406 in a9b4fa0

def throw(self, __typ: BaseException, __val: None = ..., __tb: TracebackType | None = ...) -> _T_co: ...

---

Minimal repro

(Must be run in py3.11)

>>> import asyncio
>>> async def f():
...   await asyncio.sleep(0)
>>> async def g():
...   await asyncio.gather(f(), f())
>>> h = g()
>>> h.send(None)
<stdin>:2: DeprecationWarning: There is no current event loop
<_GatheringFuture pending>
>>> h.throw(ValueError("some error"), None, None)
Traceback (most recent call last):
  ...
TypeError: throw() third argument must be a traceback

Notes:

• this does not happen if you instead do h = f(), nor if you skip the h.send(None), so there is something important about the asyncio.gather.
• if you run this in py3.10, you will get ValueError: some error, so this is a behaviour change in 3.11.
• Possibly this is caused by python/cpython@0a8b8e0 ?

[srittau]

I couldn't find anything about this in the release notes. I suspect this is actually a bug in Python 3.11.

[hauntsaninja]

Can confirm that this behaviour is changed by the commit you mention python/cpython@0a8b8e0

[hauntsaninja]

Not sure why it was changed, possibly unintentionally since the PR is missing a news entry. Let me open an issue against CPython...

[srittau]

This has now been fixed in cpython main and a backport PR for 3.11 is open. I don't think we need to do anything here. Thanks, @alicederyn for opening the issue and @hauntsaninja for fixing it in CPython!