How to start websocket server in thread using python? - python-3.x

Question

I want to start websocket server in separate thread. I have tried to implement as below but
getting Runtime error as it says attached to different loop
Code:
#!/usr/bin/env python
# WS server example
import asyncio
import websockets
import threading
import time
async def hello(websocket, path):
name = await websocket.recv()
print(name)
greeting = "Hello " + name +"!"
await websocket.send(greeting)
print(greeting)
start_server = websockets.serve(hello, "localhost", 8765)
eventLoop = asyncio.new_event_loop()
time.sleep(2)
def startWebSocket(loop, server):
print("WS: thread started")
asyncio.set_event_loop(eventLoop)
asyncio.get_event_loop().run_until_complete(start_server)
eventLoop.run_forever()
print("Run web socket in threaded env")
TH = threading.Thread(target=startWebSocket, args=[eventLoop, start_server,])
TH.start()
# then do some other work after this
Output:
Run web socket in threaded env
WS: thread started
Exception in thread Thread-1:
Traceback (most recent call last):
File "/usr/lib/python3.5/threading.py", line 914, in _bootstrap_inner
self.run()
File "/usr/lib/python3.5/threading.py", line 862, in run
self._target(*self._args, **self._kwargs)
File "threadWithWs.py", line 26, in startWebSocket
asyncio.get_event_loop().run_until_complete(start_server)
File "/usr/lib/python3.5/asyncio/base_events.py", line 387, in run_until_complete
return future.result()
File "/usr/lib/python3.5/asyncio/futures.py", line 274, in result
raise self._exception
File "/usr/lib/python3.5/asyncio/tasks.py", line 241, in _step
result = coro.throw(exc)
File "/usr/lib/python3.5/asyncio/tasks.py", line 564, in _wrap_awaitable
return (yield from awaitable.__await__())
File "/home/krunal/.local/lib/python3.5/site-packages/websockets/py35/server.py", line 13, in __await_impl__
server = await self._creating_server
File "/usr/lib/python3.5/asyncio/base_events.py", line 923, in create_server
infos = yield from tasks.gather(*fs, loop=self)
File "/usr/lib/python3.5/asyncio/futures.py", line 361, in __iter__
yield self # This tells Task to wait for completion.
RuntimeError: Task <Task pending coro=<_wrap_awaitable() running at /usr/lib/python3.5/asyncio/tasks.py:564> cb=[_run_until_complete_cb() at /usr/lib/python3.5/asyncio/base_events.py:164]> got Future <_GatheringFuture pending> attached to a different loop
Code exists with this error.
How to setup loop for websocket to start websocket server in thread?
I have followed this answer but no luck.

Answer 1

You don't have to use threads with asyncio. It's redundant.
Do something like this (I actually don't know what websocket is, I assume start_server is a coroutine \ an awaitable).
Create another async function, deputed to keep the loop alive. Before going into a while loop, spawn your task.
You may want to add an health check, using a global variable or wrapping all into a class.
async def run():
eventLoop.create_task(start_server())
while 1:
# if not health_check():
# exit()
await asyncio.sleep(1)
eventLoop.run_until_complete(run())
Do the other stuff by spawning more tasks.
To run blocking code into an asyncio loop, use eventLoop.run_in_executor(None, blocking_code): it's basically a friendly interface to threads.
As with threads, you live into the GIL.
Embrace the way asyncio do things.