[issue45996] Worse error from asynccontextmanager in Python 3.10

[Thomas Grainger]

Thomas Grainger <tagrain at gmail.com> added the comment:

or consider the trio version:

```
import logging
import trio
from contextlib import asynccontextmanager

@asynccontextmanager
async def foo():
    await trio.sleep(1)
    yield


async def test():
    async with trio.open_nursery() as n:
        f = foo()
        n.start_soon(f.__aenter__)
        n.start_soon(f.__aenter__)

trio.run(test)
```

```
Traceback (most recent call last):
  File "/home/graingert/projects/examples/bar.py", line 17, in <module>
    trio.run(test)
  File "/home/graingert/.virtualenvs/testing39/lib/python3.9/site-packages/trio/_core/_run.py", line 1932, in run
    raise runner.main_task_outcome.error
  File "/home/graingert/projects/examples/bar.py", line 15, in test
    n.start_soon(f.__aenter__)
  File "/home/graingert/.virtualenvs/testing39/lib/python3.9/site-packages/trio/_core/_run.py", line 815, in __aexit__
    raise combined_error_from_nursery
  File "/usr/lib/python3.9/contextlib.py", line 179, in __aenter__
    del self.args, self.kwds, self.func
AttributeError: args
```

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45996>
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