If I create a coro from an async iterator, then wait_for it w/ a timeout, but shielded, so it won't get canceled, and then await upon it, it returns invalid data.

See the attached test case.

The reason I do the following is to make sure that an async iterator that I have written doesn't return data early, and needs to wait till later.  If I didn't shield it, then the async iterator would get cancelled, and I don't want this.

I'd expect either correct results to be returned, or an exception to be raised, but in this case, and the docs for wait_for ( https://docs.python.org/3/library/asyncio-task.html#asyncio.wait_for ), I'd expect the correct results to be returned.


In the attached case, this is the results that I get:
$python3.7 asyncitertc.py 
3.7.5 (default, Oct 18 2019, 23:59:39) 
[Clang 7.0.2 (clang-700.1.81)]
timed out
yielding 1
results: None
getting 2: 2

I do not have python 3.8 to test with.

Thanks for the report.

Your example could be boiled down to the following:

coro = asynciter()
await coro
await coro

The second call incorrectly returns None but should raise an exception, double awaiting is a programming error.

For regular async functions it raises "RuntimeError: cannot reuse already awaited coroutine"

New changeset a96e06db77dcbd3433d39761ddb4615d7d96284a by Andrew Svetlov in branch 'master':
bpo-39386: Prevent double awaiting of async iterator (GH-18081)
https://github.com/python/cpython/commit/a96e06db77dcbd3433d39761ddb4615d7d96284a

New changeset 5cadd3fe3aead1b5bee1438dc03383d6739d4209 by Miss Islington (bot) in branch '3.8':
bpo-39386: Prevent double awaiting of async iterator (GH-18081)
https://github.com/python/cpython/commit/5cadd3fe3aead1b5bee1438dc03383d6739d4209

New changeset b76d5e9ee64b8ac861e856d8e41289c0977c75e1 by Miss Islington (bot) in branch '3.7':
bpo-39386: Prevent double awaiting of async iterator (GH-18081)
https://github.com/python/cpython/commit/b76d5e9ee64b8ac861e856d8e41289c0977c75e1

New changeset 925dc7fb1d0db85dc137afa4cd14211bf0d67414 by Nathaniel J. Smith in branch 'master':
bpo-39606: allow closing async generators that are already closed (GH-18475)
https://github.com/python/cpython/commit/925dc7fb1d0db85dc137afa4cd14211bf0d67414

New changeset 8dbdf5f275c6462bb522bcf3a29054239d72989d by Miss Islington (bot) in branch '3.8':
[3.8] bpo-39606: allow closing async generators that are already closed (GH-18475) (GH-18501)
https://github.com/python/cpython/commit/8dbdf5f275c6462bb522bcf3a29054239d72989d

New changeset f464edf3239f7867fe31c9cd238a68fb3b90feaa by Nathaniel J. Smith in branch '3.7':
bpo-39606: allow closing async generators that are already closed (GH-18475) (GH-18502)
https://github.com/python/cpython/commit/f464edf3239f7867fe31c9cd238a68fb3b90feaa