workaround tornado py38 compatibility issue

[minrk]

by setting the pre-3.8 eventloop policy as default

closes #4613

Workaround for existing releases is to add the same event policy line to your jupyter_notebook_config.py. I haven't tested, but am pretty sure it's loaded early enough:

import sys

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

[bollwyvl]

I think launchnotebook.py will also need the patch... more thoughts here

[asmca]

yes this patch works for me.
python3.8.0 on win7.

Name: jupyter Version: 1.0.0
Name: notebook Version: 6.0.2
Name: jupyter-core Version: 4.6.1

[scottwsides]

'jupyter notebook stop 8888' is not working on mac as well (for python 3.7.6)

Error is similar to others

File "/Users/swsides/chemistream/bin/jupyter-notebook", line 11, in
sys.exit(main())
File "/Users/swsides/chemistream/lib/python3.7/site-packages/jupyter_core/application.py", line 268, in launch_instance
return super(JupyterApp, cls).launch_instance(argv=argv, **kwargs)
File "/Users/swsides/chemistream/lib/python3.7/site-packages/traitlets/config/application.py", line 664, in launch_instance
app.start()
File "/Users/swsides/chemistream/lib/python3.7/site-packages/notebook/notebookapp.py", line 1910, in start
super(NotebookApp, self).start()
File "/Users/swsides/chemistream/lib/python3.7/site-packages/jupyter_core/application.py", line 257, in start
self.subapp.start()
File "/Users/swsides/chemistream/lib/python3.7/site-packages/notebook/notebookapp.py", line 466, in start
if not self.shutdown_server(server):
File "/Users/swsides/chemistream/lib/python3.7/site-packages/notebook/notebookapp.py", line 457, in shutdown_server
return shutdown_server(server, log=self.log)
File "/Users/swsides/chemistream/lib/python3.7/site-packages/notebook/notebookapp.py", line 417, in shutdown_server
HTTPClient().fetch(req)
File "/Users/swsides/chemistream/lib/python3.7/site-packages/tornado/httpclient.py", line 133, in fetch
functools.partial(self._async_client.fetch, request, **kwargs)
File "/Users/swsides/chemistream/lib/python3.7/site-packages/tornado/ioloop.py", line 532, in run_sync
return future_cell[0].result()
tornado.httpclient.HTTPClientError: HTTP 403: Forbidden

[kevin-bates]

@scottwsides - your traceback is unrelated to the py38 compatbility issue. That issue only occurs on Windows platforms with Python 3.8.

Could you please open a new issue with this information? Please also indicate if you're using a token as the authentication mechanism or a password. Thanks.