asyncio: how to set a "Proactor event loop" policy? Issue with asyncio.run()

[vstinner]

BPO	33792
Nosy	@vstinner, @asvetlov, @1st1, @miss-islington
PRs	bpo-33792: Add selector and proactor windows policies #7487 [3.7] bpo-33792: Add selector and proactor windows policies (GH-7487) #7508

^{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 2018-06-08.03:43:15.087>
created_at = <Date 2018-06-07.10:02:04.335>
labels = ['3.7', '3.8', 'expert-asyncio']
title = 'asyncio: how to set a "Proactor event loop" policy? Issue with asyncio.run()'
updated_at = <Date 2018-06-08.03:43:15.086>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2018-06-08.03:43:15.086>
actor = 'yselivanov'
assignee = 'none'
closed = True
closed_date = <Date 2018-06-08.03:43:15.087>
closer = 'yselivanov'
components = ['asyncio']
creation = <Date 2018-06-07.10:02:04.335>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 33792
keywords = ['patch']
message_count = 5.0
messages = ['318919', '318924', '318929', '319002', '319004']
nosy_count = 4.0
nosy_names = ['vstinner', 'asvetlov', 'yselivanov', 'miss-islington']
pr_nums = ['7487', '7508']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue33792'
versions = ['Python 3.7', 'Python 3.8']

[vstinner]

asyncio documentation suggets to use:
---

import asyncio, sys

if sys.platform == 'win32':
    loop = asyncio.ProactorEventLoop()
    asyncio.set_event_loop(loop)

---

https://docs.python.org/dev/library/asyncio-eventloops.html

But this code doesn't work with asyncio.run() which creates a new event loop with the current policy, and the default policy on Windows is to use SelectorEventLoop.

I cannot find a "Proactor event loop policy" in asyncio, nor how to change the default policy to use Proactor event loop.

The workaround is to not use asyncio.run() which has been added in Python 3.7.

[vstinner]

Ugly workaround:

policy = asyncio.get_event_loop_policy()
policy._loop_factory = asyncio.ProactorEventLoop

It uses a private attribute :-(

[asvetlov]

Looks like we need a proactor policy class.

[1st1]

New changeset 8f40429 by Yury Selivanov in branch 'master':
bpo-33792: Add selector and proactor windows policies (GH-7487)
8f40429

[miss-islington]

New changeset 0738443 by Miss Islington (bot) in branch '3.7':
bpo-33792: Add selector and proactor windows policies (GH-7487)
0738443