awaiting on coroutine that is being awaited should be an error

[1st1]

BPO	25888
Nosy	@gvanrossum, @brettcannon, @ncoghlan, @vstinner, @asvetlov, @vadmium, @1st1
Files	Issue25888.patch

^{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/1st1'
closed_at = <Date 2016-03-02.16:33:32.968>
created_at = <Date 2015-12-16.21:51:12.584>
labels = ['interpreter-core', 'type-feature']
title = 'awaiting on coroutine that is being awaited should be an error'
updated_at = <Date 2016-03-02.16:33:32.966>
user = 'https://github.com/1st1'

bugs.python.org fields:

activity = <Date 2016-03-02.16:33:32.966>
actor = 'yselivanov'
assignee = 'yselivanov'
closed = True
closed_date = <Date 2016-03-02.16:33:32.968>
closer = 'yselivanov'
components = ['Interpreter Core']
creation = <Date 2015-12-16.21:51:12.584>
creator = 'yselivanov'
dependencies = []
files = ['41339']
hgrepos = []
issue_num = 25888
keywords = ['patch']
message_count = 7.0
messages = ['256539', '256565', '256615', '258043', '258046', '261125', '261126']
nosy_count = 8.0
nosy_names = ['gvanrossum', 'brett.cannon', 'ncoghlan', 'vstinner', 'asvetlov', 'python-dev', 'martin.panter', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue25888'
versions = ['Python 3.5', 'Python 3.6']

[1st1]

This problem was discussed in one of the asyncio issues here: python/asyncio#288

I propose to raise a RuntimeError if an 'await' expression is used for a coroutine object that's being awaited on already. This change shouldn't impact performance in any way. It will prevent users from writing code that behaves in a very weird ways (see the github link for an example).

[gvanrossum]

Right.

[1st1]

The patch is attached, please review.

[1st1]

Will commit this patch tomorrow to 3.5 and 3.6.

It only affects coroutines (async defs): if an await expression is used on a coroutine object while it's being already awaited by another coroutine, a RuntimeError("coroutine is being awaited already") will be triggered.

[vadmium]

Missing word in a comment on the review

[python-dev]

New changeset e16084e07761 by Yury Selivanov in branch '3.5':
coroutines: Error when awaiting on coroutine that's being awaited
https://hg.python.org/cpython/rev/e16084e07761

New changeset 2218ce5c592f by Yury Selivanov in branch 'default':
Merge 3.5 (issue bpo-25888)
https://hg.python.org/cpython/rev/2218ce5c592f

[1st1]

Committed. Thanks for the review, Martin!