Default event loop policy doc lacks precision

[pitrou]

BPO	31950
Nosy	@gvanrossum, @pitrou, @vstinner, @giampaolo, @1st1
PRs	bpo-31950: Improve event loop policy doc #4306 [3.6] bpo-31950: Improve event loop policy doc (GH-4306) #4307

^{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 2017-11-07.09:35:23.197>
created_at = <Date 2017-11-05.18:16:09.678>
labels = ['type-bug', '3.7', 'docs', 'expert-asyncio']
title = 'Default event loop policy doc lacks precision'
updated_at = <Date 2017-11-07.09:35:23.195>
user = 'https://github.com/pitrou'

bugs.python.org fields:

activity = <Date 2017-11-07.09:35:23.195>
actor = 'pitrou'
assignee = 'docs@python'
closed = True
closed_date = <Date 2017-11-07.09:35:23.197>
closer = 'pitrou'
components = ['Documentation', 'asyncio']
creation = <Date 2017-11-05.18:16:09.678>
creator = 'pitrou'
dependencies = []
files = []
hgrepos = []
issue_num = 31950
keywords = ['patch']
message_count = 5.0
messages = ['305607', '305711', '305723', '305724', '305725']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'pitrou', 'vstinner', 'giampaolo.rodola', 'docs@python', 'yselivanov']
pr_nums = ['4306', '4307']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue31950'
versions = ['Python 3.6', 'Python 3.7']

[pitrou]

The doc for the default event loop policy states that """the default policy defines context as the current thread, and manages an event loop per thread that interacts with asyncio""". What it doesn't mention, though, is that you get a RuntimeError if you call get_event_loop() in a non-main thread -- you first have to call set_event_loop(new_event_loop()) in that thread. This wasn't expected by me.

As a side note, customizing the event loop policy isn't trivial either: it seems it's best to subclass DefaultEventLoopPolicy and make adjustments by overriding the API methods. Writing your own wouldn't work, as e.g. the Unix default policy has custom initialization code for child watchers.

[gvanrossum]

Agreed that this needs more clarification. Is something stopping you from submitting a PR? Note that the thing about non-main-threads is mentioned in the docstring for BaseDefaultEventLoopPolicy. I agree that the best way to create a new policy is to study the existing code and subclass DefaultEventLoopPolicy.

[pitrou]

New changeset 4135c89 by Antoine Pitrou in branch 'master':
bpo-31950: Improve event loop policy doc (bpo-4306)
4135c89

[pitrou]

New changeset e65617f by Antoine Pitrou (Miss Islington (bot)) in branch '3.6':
bpo-31950: Improve event loop policy doc (GH-4306) (bpo-4307)
e65617f

[pitrou]

Thank you Guido. This is fixed now.