Event loop implementation docs advertise set_event_loop which doesn't work with asyncio.run

[hniksic]

BPO	38423
Nosy	@hniksic, @asvetlov, @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 = None
created_at = <Date 2019-10-09.17:14:39.175>
labels = ['3.8', 'docs', '3.7', '3.9', 'expert-asyncio']
title = "Event loop implementation docs advertise set_event_loop which doesn't work with asyncio.run"
updated_at = <Date 2020-01-27.15:18:48.508>
user = 'https://github.com/hniksic'

bugs.python.org fields:

activity = <Date 2020-01-27.15:18:48.508>
actor = 'Cilyan Olowen'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'asyncio']
creation = <Date 2019-10-09.17:14:39.175>
creator = 'hniksic'
dependencies = []
files = []
hgrepos = []
issue_num = 38423
keywords = []
message_count = 4.0
messages = ['354288', '354289', '360770', '360772']
nosy_count = 5.0
nosy_names = ['hniksic', 'asvetlov', 'docs@python', 'yselivanov', 'Cilyan Olowen']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue38423'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[hniksic]

The docs of SelectorEventLoop and ProactorEventLoop contain examples that call asyncio.set_event_loop:

      selector = selectors.SelectSelector()
      loop = asyncio.SelectorEventLoop(selector)
      asyncio.set_event_loop(loop)

But this won't have any effect on code that uses asyncio.run(), because asyncio.run() creates a fresh event loop. Since asyncio.run() is the recommended way to execute async code and is used consistently throughout the documentation, this might be confusing to someone who tries to e.g. use the proactor loop on Windows.

I propose the following:

• add a disclaimer that instantiating the event loop won't affect calls to asyncio.run(), and that loop.run_until_complete() must be used; and

• link to asyncio.set_event_loop_policy(), which does work with asyncio.run(), but doesn't allow fine-tuning the details, such as which selector to use for the SelectorEventLoop.

[1st1]

Yes, docs updates sound good.

[CilyanOlowen]

It doesn't seem to work either for asyncio.get_event_loop, especially when using asyncio.wait(), the loop complains that

"RuntimeError: Task <Task pending coro=<Event.wait() running at lib\asyncio\locks.py:293> cb=[_wait.<locals>._on_completion() at lib\asyncio\tasks.py:440]> got Future <Future pending> attached to a different loop"

[CilyanOlowen]

My bad, I did not check that asyncio.Event() was called before the application has a chance to properly set_event_loop with custom one.