Python asyncio event loop. Run loop forever after completing an async task

Question

Well I'm new to async in python. I'm creating a server using the call asyncio.start_server , the problem is that I'm running the same loop twice, the first time to create/start the server calling the loop.run_until_complete , and after that loop.run_forever . Here the code I use.

if __name__ == '__main__':
    loop = asyncio.get_event_loop()

    sv_wrapper = ServerWrapper(
        host='localhost',
        port=5003
    )

    loop.run_until_complete(sv_wrapper.create())
    print(repr(sv_wrapper.server))

    loop.run_forever()

( Full code example )
Honestly I do not get the last call to loop.run_forever() , does de created server with asyncio.start_server run on the same event loop that executes the call, or a new event loop is created internally?

If a new event loop is created internally, I do not need the call to run forever, for example just keeping the process running could be enough (and of course having a reference to the created Server ).

I do not know if this have sense at all, but if the server is a loop itself (manage in/out coming connections as future tasks) Is it possible to push tasks with loop.create_task ?

I did not come with a specific problem and sorry about that. I come from a nodejs background and I thought it would be easier to get async in python, thanks for your help, and any extras will be well received!

Answer 1

Honestly I do not get the last call to loop.run_forever(), does de created server with asyncio.start_server run on the same event loop that executes the call, or a new event loop is created internally?

It's single global event loop. Usually end-user manages creating and running event loops, libraries don't do it internally.

If a new event loop is created internally, I do not need the call to run forever, for example just keeping the process running could be enough (and of course having a reference to the created Server).

I'm not sure I understand what you mean, but here some thoughts:

1. Your server can work only while event loop is running. Server can recieve or send something only via event loop.

2. loop.run_until_complete(sv_wrapper.create()) means that event loop used to execute one job (to create server) and then being stopped. Again it means you should run it to make created server work.

I do not know if this have sense at all, but if the server is a loop itself (manage in/out coming connections as future tasks) Is it possible to push tasks with loop.create_task?

Server is not an event loop itself. Roughly saying, server is one of async tasks managed by global event loop.

You can create other async tasks ( better to do it with ensure_future ) that will be managed by same global event loop (and through that being run concurrently with server).

Answer 2

Why don't you just check the source code?

try:
    events._set_running_loop(self)
    while True:                   # Here is the point.
        self._run_once()          # Run event loop once.
        if self._stopping:        # Check stop
            break                 # Stop event loop.
finally:
    self._stopping = False
    self._thread_id = None
    events._set_running_loop(None)
    self._set_coroutine_wrapper(False)
    if self._asyncgens is not None:
        sys.set_asyncgen_hooks(*old_agen_hooks)

This is part of run_forever . You can see, without call run_forever , you will not even run any task.