python 3.8 changes default asyncio loop on windows https://bugs.python.org/issue37373

[Savier]

See https://bugs.python.org/issue37373
I failed to find failing tests in tornado, but loop's add_reader() now "not implemented" by default on windows and asyncio platform uses it.

Here is the crash form Jupyter notebook:

C:\Python38\Scripts>jupyter-notebook.exe
Traceback (most recent call last):
  File "c:\python38\lib\runpy.py", line 192, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "c:\python38\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Python38\Scripts\jupyter-notebook.exe\__main__.py", line 9, in <module>
  File "c:\python38\lib\site-packages\jupyter_core\application.py", line 267, in launch_instance
    return super(JupyterApp, cls).launch_instance(argv=argv, **kwargs)
  File "c:\python38\lib\site-packages\traitlets\config\application.py", line 663, in launch_instance
    app.initialize(argv)
  File "<c:\python38\lib\site-packages\decorator.py:decorator-gen-7>", line 2, in initialize
  File "c:\python38\lib\site-packages\traitlets\config\application.py", line 87, in catch_config_error
    return method(app, *args, **kwargs)
  File "c:\python38\lib\site-packages\notebook\notebookapp.py", line 1679, in initialize
    self.init_webapp()
  File "c:\python38\lib\site-packages\notebook\notebookapp.py", line 1442, in init_webapp
    self.http_server.listen(port, self.ip)
  File "c:\python38\lib\site-packages\tornado\tcpserver.py", line 152, in listen
    self.add_sockets(sockets)
  File "c:\python38\lib\site-packages\tornado\tcpserver.py", line 165, in add_sockets
    self._handlers[sock.fileno()] = add_accept_handler(
  File "c:\python38\lib\site-packages\tornado\netutil.py", line 279, in add_accept_handler
    io_loop.add_handler(sock, accept_handler, IOLoop.READ)
  File "c:\python38\lib\site-packages\tornado\platform\asyncio.py", line 99, in add_handler
    self.asyncio_loop.add_reader(fd, self._handle_events, fd, IOLoop.READ)
  File "c:\python38\lib\asyncio\events.py", line 501, in add_reader
    raise NotImplementedError
NotImplementedError

I think patch is trivial:
tornado/platform/asyncio.py :

import sys

if sys.platform == 'win32':
    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())

But it's too hard to me is to write tests.

There is a note on the top of tornado/platform/asyncio.py describing situation, but it seems it pretends application must set such policy. I don't know which way is correct, my patch solves the problem for me, I think devs must communicate in all of three places - here, jupyter issues, python bugs to make a decision.

[mivade]

#2608 has some discussions about why Tornado should just document what to do on Windows rather than make a decision for users.

[bdarnell]

Closing as a duplicate of #2608

[Priteshkamde]

For Django, in Windows 10, Python : 3.8.1
Add this in settings.py

import sys
import asyncio

if sys.platform == 'win32':
    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())