"not implemented" message with jupyter notebook under python-3.8.0a4 (due to switch to "proactor" loop on windows)

[stonebig]

I have this strange message with Tornado-5.1.1 and Jupyter Notebook

Traceback (most recent call last):
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\runpy.py", line 192, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\scripts\jupyter-notebook.exe\__main__.py", line 9, in <module>
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\site-packages\jupyter_core\application.py", line 266, in launch_instance
    return super(JupyterApp, cls).launch_instance(argv=argv, **kwargs)
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\site-packages\traitlets\config\application.py", line 657, in launch_instance
    app.initialize(argv)
  File "<C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\site-packages\decorator.py:decorator-gen-7>", line 2, in initialize
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\site-packages\traitlets\config\application.py", line 87, in catch_config_error
    return method(app, *args, **kwargs)
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\site-packages\notebook\notebookapp.py", line 1630, in initialize
    self.init_webapp()
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\site-packages\notebook\notebookapp.py", line 1407, in init_webapp
    self.http_server.listen(port, self.ip)
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\site-packages\tornado\tcpserver.py", line 144, in listen
    self.add_sockets(sockets)
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\site-packages\tornado\tcpserver.py", line 157, in add_sockets
    self._handlers[sock.fileno()] = add_accept_handler(
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\site-packages\tornado\netutil.py", line 268, in add_accept_handler
    io_loop.add_handler(sock, accept_handler, IOLoop.READ)
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\site-packages\tornado\platform\asyncio.py", line 79, in add_handler
    self.asyncio_loop.add_reader(
  File "C:\WinP\bd38\bu\WPy64-3800a4\python-3.8.0a4.amd64\lib\asyncio\events.py", line 499, in add_reader
    raise NotImplementedError
NotImplementedError

Does anyone as a clue of the problem ? is it a Tornado, a Notebook, a Python-3.8.0a4 bug (or feature) ?

[kevin-bates]

Life on the edge - this appears to be a side-affect of Python 3.8 on Windows: tornadoweb/tornado#2608.

[stonebig]

yes, Python masters do alpha of python-3.8.0.a4, so I suppose it is to use it for some blood-bath testing, like a good episode of GOT.

[stonebig]

and so apparently, Tornado has zero intention to use the "Proactor" event loop.

Maybe Jupyter Notebook has to implement a change ? or ask Python asyncio masters to revert to previous default ?

[jasongrout]

Apparently progress is at tornadoweb/tornado#2656 (comment)

[stonebig]

and there may be also further impact on "async exec", if it lands in Python-3.8.0b1 https://bugs.python.org/issue34616

[stonebig]

so @jasongrout , as Tornado team doesn't whish to enforce this, it seems the Jupyter applications have to replace in all its packages that can launch first the asyncio loop :

import asyncio

per

import asyncio
import sys
if sys.platform == 'win32' and sys.version_info > (3, 8, 0, 'alpha', 3) :
    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())

[rolweber]

@stonebig What if users want to configure another event loop policy, as mentioned here: #4184 (comment)

How about calling asyncio.get_event_loop_policy() to check if the problematic Windows policy is actually in effect, and only setting a different one in that case?

[stonebig]

@rolweber my "hard way" fix is for sure lacking in subtlety. it's alpha4 answer to alpha4 problem.

[stonebig]

ah, maybe hope from another direction https://bugs.python.org/issue29883

[stonebig]

I find it's apparently a limitation of 0mq ... https://python-tulip.narkive.com/x36hFXG2/proactoreventloop-and-add-reader-add-writer

[stonebig]

latest news : https://bugs.python.org/issue37373

[stonebig]

Apparently there is an undocumentd solution python-trio/trio#52, and Nathaniel looks enthousiastic python-trio/trio#52 (comment)