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.

Ugly workaround:

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

It uses a private attribute :-(

Looks like we need a proactor policy class.

New changeset 8f4042964d5b0ddf5cdf87862db962ba64e3f64a by Yury Selivanov in branch 'master':
bpo-33792: Add selector and proactor windows policies (GH-7487)
https://github.com/python/cpython/commit/8f4042964d5b0ddf5cdf87862db962ba64e3f64a

New changeset 0738443a5b071a6bd2c18957a06cfe571a7314f2 by Miss Islington (bot) in branch '3.7':
bpo-33792: Add selector and proactor windows policies (GH-7487)
https://github.com/python/cpython/commit/0738443a5b071a6bd2c18957a06cfe571a7314f2