asyncio: document behaviour of wait() cancellation

[vstinner]

BPO	23859
Nosy	@gvanrossum, @vstinner, @cjerdonek, @1st1
PRs	bpo-23859: Document that asyncio.wait() does not cancel its futures #7217 [3.7] bpo-23859: Document that asyncio.wait() does not cancel its futures (GH-7217) #7227 [3.6] bpo-23859: Document that asyncio.wait() does not cancel its futures (GH-7217) #7228 bpo-23859: Fix typo introduced in GH-7217 #7230

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

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GitHub fields:

assignee = None
closed_at = <Date 2018-07-11.21:46:40.835>
created_at = <Date 2015-04-03.15:13:17.440>
labels = ['3.7', '3.8', 'expert-asyncio']
title = 'asyncio: document behaviour of wait() cancellation'
updated_at = <Date 2018-07-11.21:46:40.834>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2018-07-11.21:46:40.834>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2018-07-11.21:46:40.835>
closer = 'vstinner'
components = ['asyncio']
creation = <Date 2015-04-03.15:13:17.440>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 23859
keywords = ['patch']
message_count = 5.0
messages = ['240001', '318124', '318136', '318145', '318161']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'vstinner', 'chris.jerdonek', 'yselivanov']
pr_nums = ['7217', '7227', '7228', '7230']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue23859'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[vstinner]

When wait(fs) is cancelled, futures of fs are not cancelled. This behaviour is not documented. It should be documented, and a warning should be added to the Cancellation section:
https://docs.python.org/dev/library/asyncio-dev.html#cancellation

See my message "Cancellation of asyncio.wait()":
https://mail.google.com/mail/u/0/#label/Tulip/14b34f0a5d31114f

[1st1]

New changeset f9aeca2 by Yury Selivanov (Elvis Pranskevichus) in branch 'master':
bpo-23859: Document that asyncio.wait() does not cancel its futures (bpo-7217)
f9aeca2

[vstinner]

f9aeca2

" + when a timeout accurs. "

typo: accurs => occurs

[1st1]

New changeset f952694 by Yury Selivanov (Miss Islington (bot)) in branch '3.7':
[3.7] bpo-23859: Document that asyncio.wait() does not cancel its futures (GH-7217) (bpo-7227)
f952694

[vstinner]

Thank you Elvis for the documentation! Cancellation is one of the most complex part of asynchronous programming, and I don't think that asyncio documentation explains it properly. We may document the behaviour on cancellation for most task functions.

https://docs.python.org/dev/library/asyncio-task.html#task-functions

as_completed() behaviour on cancellation is not documented for example.

The wait() documentation should be backported to 3.6 as well.