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 set

CPython versions tested on:

3.11, 3.12, 3.13

Operating systems tested on:

macOS

Linked PRs

• gh-128078: Clear exception in anext before calling _PyGen_SetStopIterationValue #128780
• [3.12] gh-128078: Clear exception in anext before calling _PyGen_SetStopIterationValue (GH-128780) #128784
• [3.13] gh-128078: Clear exception in anext before calling _PyGen_SetStopIterationValue (GH-128780) #128785
• gh-128078: Simplify logic in _PyGen_SetStopIterationValue (follow-up to gh-128780) #128287
• [3.13] gh-128078: Use PyErr_SetRaisedException in _PyGen_SetStopIterationValue (GH-128287) #128789
• [3.12] gh-128078: Use PyErr_SetRaisedException in _PyGen_SetStopIterationValue (GH-128287) #128790