[BUG] Webhook NotImplementedError on windows with python 3.8

[n5y]

Steps to reproduce

from telegram.ext import Updater
updater = Updater(TOKEN, use_context=True)
updater.start_webhook()
updater.idle()

Expected behaviour

No console output

Actual behaviour

An exception is logged

Exception in thread Bot:377489430:updater:
Traceback (most recent call last):
  File "C:\Program Files\Python38\lib\threading.py", line 932, in _bootstrap_inner
    self.run()
  File "C:\Program Files\Python38\lib\threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\User\.virtualenvs\sandbox38\lib\site-packages\telegram\ext\updater.py", line 216, in _thread_wrapper
    target(*args, **kwargs)
  File "C:\Users\User\.virtualenvs\sandbox38\lib\site-packages\telegram\ext\updater.py", line 452, in _start_webhook
    self.httpd.serve_forever()
  File "C:\Users\User\.virtualenvs\sandbox38\lib\site-packages\telegram\utils\webhookhandler.py", line 52, in serve_forever
    self.http_server.listen(self.port, address=self.listen)
  File "C:\Users\User\.virtualenvs\sandbox38\lib\site-packages\tornado\tcpserver.py", line 152, in listen
    self.add_sockets(sockets)
  File "C:\Users\User\.virtualenvs\sandbox38\lib\site-packages\tornado\tcpserver.py", line 165, in add_sockets
    self._handlers[sock.fileno()] = add_accept_handler(
  File "C:\Users\User\.virtualenvs\sandbox38\lib\site-packages\tornado\netutil.py", line 279, in add_accept_handler
    io_loop.add_handler(sock, accept_handler, IOLoop.READ)
  File "C:\Users\User\.virtualenvs\sandbox38\lib\site-packages\tornado\platform\asyncio.py", line 100, in add_handler
    self.asyncio_loop.add_reader(fd, self._handle_events, fd, IOLoop.READ)
  File "C:\Program Files\Python38\lib\asyncio\events.py", line 501, in add_reader
    raise NotImplementedError
NotImplementedError

The process continues to run, trying to interrupt it results in a different exception being logged.

Configuration

Operating System: Windows 10

Version of Python, python-telegram-bot & dependencies:

certifi 2020.04.05.1
future 0.18.2
Python 3.8.3 (tags/v3.8.3:6f8c832, May 13 2020, 22:37:02) [MSC v.1924 64 bit (AMD64)]

See also

tornadoweb/tornado#2608
https://bugs.python.org/issue37373

[Bibo-Joshi]

Hi. Thanks for filing the issue. I haven't looked into the details yet, but I see that I referenced the tornado issue in #1614, where a workaround was added. Can you confirm that the issue persists with ptb>=12.5 (#1614 was released in 12.5)?

[n5y]

Sorry, I somehow missed the ptb version in that last block.

python-telegram-bot 12.7
certifi 2020.04.05.1
future 0.18.2
Python 3.8.3 (tags/v3.8.3:6f8c832, May 13 2020, 22:37:02) [MSC v.1924 64 bit (AMD64)]

Yes, the issue is the issue persists with ptb>=12.5

[n5y]

The reason the workaround doesn't work is because it changes event loop policy after an event loop was already created. So one way to fix this is to apply it earlier.

Event loop policy is a global setting, changing it can break user code, and must be done as soon as possible (if at all). A library generally shouldn't change it. If we must change event loop policy, the change should not be hidden form the user. I believe it would be appropriate to set event loop policy in start_webhook and document this in its docstring.

Alternative solution is to create a selector event loop instance specifically for tornado without changing the global event loop policy. asyncio.set_event_loop(asyncio.SelectorEventLoop()) This partially sidesteps the usual event loop policy execution, and therefore might have non obvious implication in non obvious scenarios, but the system + python version + event loop policy check used in original workaround should make this very safe.

[prdaneshi]

i got the same problem and used
import asyncio asyncio.set_event_loop(asyncio.SelectorEventLoop()) to solved it.