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Async generator/anext with default-tuple-value results in SystemError: <class 'StopIteration'> returned with exception set #128078
Description
Bug report
Bug description:
I asked a question on stackoverflow because I found a strange error while writing code that makes use an async generator function and the built-in anext() function when using a tuple value for the default argument of anext().
One of the stackoverflow answers contains a good analysis and a possible cause of the problem.
The failing code is the second line with anext():
import asyncio
async def generator(it=None):
if it is not None:
yield (it, it)
async def my_func():
# results in a=1 b=1
a, b = await anext(generator(1), (2, 3))
# results in no printing
async for a, b in generator():
print(a, b)
# raises exception
a, b = await anext(generator(), (2, 3))
loop = asyncio.new_event_loop()
loop.run_until_complete(my_func())It raises this unexpected exception:
StopAsyncIteration
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/manuel/.pyenv/versions/3.12.0/lib/python3.12/asyncio/base_events.py", line 664, in run_until_complete
return future.result()
^^^^^^^^^^^^^^^
File "<stdin>", line 6, in my_func
SystemError: <class 'StopIteration'> returned a result with an exception setCPython versions tested on:
3.11, 3.12, 3.13
Operating systems tested on:
macOS
Linked PRs
- gh-128078: Clear exception in
anextbefore calling_PyGen_SetStopIterationValue #128780 - [3.12] gh-128078: Clear exception in
anextbefore calling_PyGen_SetStopIterationValue(GH-128780)Â #128784 - [3.13] gh-128078: Clear exception in
anextbefore calling_PyGen_SetStopIterationValue(GH-128780)Â #128785 - gh-128078: Simplify logic in
_PyGen_SetStopIterationValue(follow-up to gh-128780)Â #128287 - [3.13] gh-128078: Use
PyErr_SetRaisedExceptionin_PyGen_SetStopIterationValue(GH-128287)Â #128789 - [3.12] gh-128078: Use
PyErr_SetRaisedExceptionin_PyGen_SetStopIterationValue(GH-128287)Â #128790
Activity
kumaraditya303 commentedon Jan 11, 2025
FWIW I think its a bug in implementation of anext.
Specifically in these lines:
cpython/Objects/iterobject.c
Lines 386 to 388 in 3a570c6
This should clear the
PyExc_StopAsyncIterationbefore calling_PyGen_SetStopIterationValue.The following patch fixes it for me and all async gen tests pass:
picnixz commentedon Jan 11, 2025
This is something I've also wondered. We could clear the exception before calling
_PyGen_SetStopIterationValuebut Mark also suggested removing the specialized path for tuples. So we can perhaps do both in my PR?kumaraditya303 commentedon Jan 11, 2025
I suggest to do both in different PRs, I would prefer to backport the simpler fix.
picnixz commentedon Jan 11, 2025
Doing both is fine. I'm not on my dev session anymore but if you want to do the
PyErr_Clear()part, it's better to merge it first. Then I'll merge mine where I can just check that no exception is currently set instead.gh-128078: Clear exception in `anext` before calling `_PyGen_SetStopIâĶ
pythongh-128078: Clear exception in `anext` before calling `_PyGen_SeâĶ
pythongh-128078: Clear exception in `anext` before calling `_PyGen_SeâĶ
[3.12] gh-128078: Clear exception in `anext` before calling `_PyGen_SâĶ
[3.13] gh-128078: Clear exception in `anext` before calling `_PyGen_SâĶ
gh-128078: Use `PyErr_SetRaisedException` in `_PyGen_SetStopIterationâĶ
pythongh-128078: Use `PyErr_SetRaisedException` in `_PyGen_SetStopIteâĶ
pythongh-128078: Use `PyErr_SetRaisedException` in `_PyGen_SetStopIteâĶ
[3.12] gh-128078: Use `PyErr_SetRaisedException` in `_PyGen_SetStopItâĶ
[3.13] gh-128078: Use `PyErr_SetRaisedException` in `_PyGen_SetStopItâĶ