Allow to create new Event Loops on Threads

[chexca]

BPO	40811
Nosy	@tiran, @asvetlov, @1st1, @iritkatriel, @chexca
PRs	bpo-40811: Allowing to create event loops on threads #20500

^{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 2022-03-17.13:16:16.702>
created_at = <Date 2020-05-28.17:54:39.337>
labels = ['type-bug', '3.10', 'expert-asyncio']
title = 'Allow to create new Event Loops on Threads'
updated_at = <Date 2022-03-17.13:16:16.701>
user = 'https://github.com/chexca'

bugs.python.org fields:

activity = <Date 2022-03-17.13:16:16.701>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2022-03-17.13:16:16.702>
closer = 'asvetlov'
components = ['asyncio']
creation = <Date 2020-05-28.17:54:39.337>
creator = 'Christian Exposito'
dependencies = []
files = []
hgrepos = []
issue_num = 40811
keywords = ['patch']
message_count = 4.0
messages = ['370257', '370261', '393959', '415410']
nosy_count = 6.0
nosy_names = ['christian.heimes', 'asvetlov', 'python-dev', 'yselivanov', 'iritkatriel', 'Christian Exposito']
pr_nums = ['20500']
priority = 'normal'
resolution = 'wont fix'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40811'
versions = ['Python 3.10']

[chexca]

Right now, Async IO module is not automatically creating a new event loop on threads that are not the main thread (https://github.com/python/cpython/blob/master/Lib/asyncio/events.py#L638), but it should be interesting to do it. For example, WSGI applications handle web requests by spawning a new thread. If we allow Async IO module to create event loops on those threads, we will improve its adoption for developing web applications.

[tiran]

The default policy of asyncio deliberately limits automatic creation of event loops to the main thread. You can't just mix asyncio with a threaded web application and expect it to work correctly.

See

cpython/Lib/asyncio/events.py

Lines 609 to 620 in a487a39

	class BaseDefaultEventLoopPolicy(AbstractEventLoopPolicy):
	"""Default policy implementation for accessing the event loop.
	In this policy, each thread has its own event loop. However, we
	only automatically create an event loop by default for the main
	thread; other threads by default have no event loop.
	Other policies may have different rules (e.g. a single global
	event loop, or automatically creating an event loop per thread, or
	using some other notion of context to which an event loop is
	associated).
	"""

In this policy, each thread has its own event loop.  However, we
only automatically create an event loop by default for the main
thread; other threads by default have no event loop.

Other policies may have different rules (e.g. a single global
event loop, or automatically creating an event loop per thread, or
using some other notion of context to which an event loop is
associated).

[iritkatriel]

Should this be closed as rejected or is there further discussion?

[asvetlov]

I suggest closing.
The documentation explicitly describes how asyncio works with threads.