asyncio: add support for async context manager on streams?

[vstinner]

BPO	26373
Nosy	@gvanrossum, @vstinner, @vadmium, @1st1

^{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 2017-06-28.01:11:31.754>
created_at = <Date 2016-02-17.11:30:21.910>
labels = ['type-feature', 'expert-asyncio']
title = 'asyncio: add support for async context manager on streams?'
updated_at = <Date 2017-06-28.01:11:31.752>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2017-06-28.01:11:31.752>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2017-06-28.01:11:31.754>
closer = 'vstinner'
components = ['asyncio']
creation = <Date 2016-02-17.11:30:21.910>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 26373
keywords = []
message_count = 4.0
messages = ['260394', '267201', '267243', '297105']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'vstinner', 'python-dev', 'martin.panter', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue26373'
versions = ['Python 3.6']

[vstinner]

While working on the issue bpo-24911 (Context manager of socket.socket is not documented), I recalled that asyncio objects don't support context manager. With the PEP-492, it becomes possible to add support for async context manager for a few asyncio objects. I suggest to start with StreamWriter.

Usually, my rationale to decide which object should implement context manager is to check if the destructor can emit ResourceWarning.

In asyncio, ResourceWarning is now emitted by transport and event loop destructors:
https://docs.python.org/dev/library/asyncio-dev.html#close-transports-and-event-loops

[python-dev]

New changeset 883cfb3e28f9 by Gregory P. Smith in branch '3.5':
Fixes Issue bpo-26373: subprocess.Popen.communicate now correctly ignores
https://hg.python.org/cpython/rev/883cfb3e28f9

New changeset 78e81de6d447 by Gregory P. Smith in branch 'default':
merge from 3.5 - Fixes Issue bpo-26373: subprocess.Popen.communicate
https://hg.python.org/cpython/rev/78e81de6d447

[vadmium]

Above two commits were actuall for bpo-26372

[vstinner]

I'm not sure asynico anymore and lost track of this issue, so I just close it.