Make event loops with statement context managers

[MathiasFrjdman]

BPO	24795
Nosy	@gvanrossum, @vstinner, @asvetlov, @vadmium, @1st1
Files	patch: Patch against python 3.5b4 asyncio/base_events.py

^{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 2017-12-20.21:18:01.081>
created_at = <Date 2015-08-05.12:38:28.664>
labels = ['type-feature', 'expert-asyncio']
title = 'Make event loops with statement context managers'
updated_at = <Date 2017-12-20.21:18:01.080>
user = 'https://bugs.python.org/MathiasFrjdman'

bugs.python.org fields:

activity = <Date 2017-12-20.21:18:01.080>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2017-12-20.21:18:01.081>
closer = 'asvetlov'
components = ['asyncio']
creation = <Date 2015-08-05.12:38:28.664>
creator = 'Mathias Fr\xc3\xb6jdman'
dependencies = []
files = ['40129']
hgrepos = []
issue_num = 24795
keywords = []
message_count = 7.0
messages = ['248032', '248034', '248035', '248087', '248090', '248116', '308820']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'vstinner', 'asvetlov', 'martin.panter', 'yselivanov', 'Mathias Fr\xc3\xb6jdman']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue24795'
versions = ['Python 3.5']

[MathiasFrjdman]

Since asyncio event loops have to be closed nowadays, it would be pretty convenient and pythonic to make BaseEventLoop a context manager that calls self.close() in __exit__ the same way as contextlib.closing() does it. Example:

import asyncio

with asyncio.get_event_loop() as loop:
    loop.run_until_complete(func())

instead of

import asyncio
from contextlib import closing

with closing(asyncio.get_event_loop()) as loop:
    loop.run_until_complete(func())

or event the bulkier

import asyncio

loop = asyncio.get_event_loop()
try:
    loop.run_until_complete(func())
finally:
    loop.close()

The attached patch applies to Python 3.5b4's asyncio/base_events.py

[MathiasFrjdman]

(Just noticed http://bugs.python.org/issue19860, which I originally failed to notice when just searching for "asyncio loop" and not context manager)

Anyway, in recent Python/asyncio versions, failing to close the event loop before exiting whole the process can cause problems, so I think the case is valid now.

[gvanrossum]

This seems the wrong idea to me. Event loops should be long-lived, so the
context manager would ideally see very little use.

[vstinner]

+1 for me. Asyncio examples already have this try/finally pattern. I
already proposed to support context manager some months ago.

Guido, I don't understand your point. Usually the main function id
loop.run_until_complete/.run_forever. That's all. It doesn't mean that the
loop only runs a few milliseconds.

[vadmium]

From what I can see, the examples in the current documentation tend to diectly call loop.close() without an exception handler. Only two examples have the bare-bones try / finally handler (which is important for the example that uses Ctrl+C).

[gvanrossum]

My worry is that the context manager will make people believe it's a good
pattern to create an event loop just to make one call. If tests violate
this pattern, add a context manager helper function to test_utils.py.

On Thu, Aug 6, 2015 at 2:57 AM, Martin Panter <report@bugs.python.org>
wrote:

Martin Panter added the comment:

>From what I can see, the examples in the current documentation tend to
diectly call loop.close() without an exception handler. Only two examples
have the bare-bones try / finally handler (which is important for the
example that uses Ctrl+C).

----------
nosy: +vadmium

---

Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue24795\>

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[asvetlov]

Superseded by asyncio.run() function.

P.S.
Context manager is not a solution because loop.shutdown_asyncgens() should be called before loop.close() and the method is a coroutine.