AsyncGenerator breaks when not iterated fully with RuntimeError("can't send non-None value to a just-started coroutine")

[MichaelYoo]

BPO	38013
Nosy	@ned-deily, @asvetlov, @ambv, @1st1, @miss-islington, @tirkarthi
PRs	bpo-38013: Fix runtime error on closing async generator #16061 bpo-38013: make async_generator_athrow object tolerant to throwing exceptions #16070 [3.8] bpo-38013: make async_generator_athrow object tolerant to throwing exceptions (GH-16070) #16231 [3.7] bpo-38013: make async_generator_athrow object tolerant to throwing exceptions (GH-16070) #16232

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = 'https://github.com/asvetlov'
closed_at = <Date 2019-09-17.13:20:51.172>
created_at = <Date 2019-09-03.00:13:42.963>
labels = ['type-bug', '3.8', '3.9', 'release-blocker', '3.7', 'expert-asyncio']
title = 'AsyncGenerator breaks when not iterated fully with RuntimeError("can\'t send non-None value to a just-started coroutine")'
updated_at = <Date 2019-09-17.13:20:51.171>
user = 'https://bugs.python.org/MichaelYoo'

bugs.python.org fields:

activity = <Date 2019-09-17.13:20:51.171>
actor = 'asvetlov'
assignee = 'asvetlov'
closed = True
closed_date = <Date 2019-09-17.13:20:51.172>
closer = 'asvetlov'
components = ['asyncio']
creation = <Date 2019-09-03.00:13:42.963>
creator = 'Michael Yoo'
dependencies = []
files = []
hgrepos = []
issue_num = 38013
keywords = ['patch', '3.7regression']
message_count = 7.0
messages = ['351044', '352486', '352598', '352603', '352634', '352636', '352637']
nosy_count = 7.0
nosy_names = ['ned.deily', 'asvetlov', 'lukasz.langa', 'yselivanov', 'miss-islington', 'xtreak', 'Michael Yoo']
pr_nums = ['16061', '16070', '16231', '16232']
priority = 'release blocker'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue38013'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[MichaelYoo]

Version: Python 3.7.3

When I run this code:

import asyncio


class TestAsyncGenerator:
    def __init__(self):
        pass

    async def aiter(self):
        yield 1
        yield 2


async def main():
    gen = TestAsyncGenerator()
    async for number in gen.aiter():
        break


asyncio.run(main())

# unhandled exception during asyncio.run() shutdown
# task: <Task finished coro=<<async_generator_athrow without __name__>()> exception=RuntimeError("can't send non-None value to a just-started coroutine")>
# RuntimeError: can't send non-None value to a just-started coroutine

There is a warning message that I don't expect to see. I would expect breaking from an async iteration to behave as if breaking from a normal iteration - that is, no problems.

However, I see the warning message shown above. Am I missing something? Otherwise, I believe this is a bug. Thanks!

[asvetlov]

I have a PR for the fix.
Yuri preliminary agrees with the patch but he requested more time for careful review.

I've added 'release blocker' priority to don't miss this bugfix for the next release.

[ned-deily]

I've added 'release blocker' priority to don't miss this bugfix for the next release.

Note that 3.7.5rc1 is scheduled to be tagged today so we need to make a decision on whether to hold the release for a fix. From a quick test, it looks like the behavior was introduced in 3.7.1 as the test does not fail for me with 3.7.0. If so, that seems to meet the criteria for a maintenance release regression and thus potentially a release blocker.

[asvetlov]

Thanks, Ned.
I love to include this in 3.7.5.
I've pinged Yuri offline with ask for review

[miss-islington]

New changeset c275312 by Miss Islington (bot) (Andrew Svetlov) in branch 'master':
bpo-38013: make async_generator_athrow object tolerant to throwing exceptions (GH-16070)
c275312

[miss-islington]

New changeset fc022f0 by Miss Islington (bot) in branch '3.7':
bpo-38013: make async_generator_athrow object tolerant to throwing exceptions (GH-16070)
fc022f0

[miss-islington]

New changeset 3c1786f by Miss Islington (bot) in branch '3.8':
bpo-38013: make async_generator_athrow object tolerant to throwing exceptions (GH-16070)
3c1786f