asyncio - how to stop loop?

[Ian Kelly]

On Wed, Jun 11, 2014 at 1:19 AM, Frank Millman <frank at chagford.com> wrote:
> First attempt - same as before
>
>     loop = asyncio.get_event_loop()
>     threading.Thread(target=loop.run_forever).start()
>     input('Press <enter> to stop')
>     loop.stop()
>     loop.close()

Each event loop is hosted by a specific thread.  In this case you're
getting the event loop of the main thread and then trying to run it in
a separate thread, which is not a good idea.  You can run an event
loop in a separate thread, but you should install a separate event
loop for that thread if you do (and then when you interact with the
loop, do so in a thread-safe manner -- see below).

> Second attempt - move the keyboard input to a separate thread
>
>     def stop_loop():
>         input('Press <enter> to stop')
>         loop.stop()
>         loop.close()
>
>     loop = asyncio.get_event_loop()
>     threading.Thread(target=stop_loop).start()
>     loop.run_forever()

One issue here is that (like most event loop implementations) event
loops are not thread-safe.  To make a call to the event loop across
threads, you should be using the call_soon_threadsafe method, e.g.
"loop.call_soon_threadsafe(loop.stop)".  You'll also want to make sure
that the event loop has actually stopped before you call loop.close --
see below.

> Third attempt - get the loop to close itself (cannot use in practice, but
> see what happens)
>
>     def stop_loop():
>         loop.stop()
>         loop.close()
>
>     loop = asyncio.get_event_loop()
>     loop.call_later(2, stop_loop)
>     loop.run_forever()

I think what's happening here is that per the docs loop.close should
not be called while the loop is running.  You've called loop.stop but
you're still inside a callback, which is a bit of a gray area.  You
probably don't need to call loop.close at all, but if you want to do
so I suggest putting it after the run_forever call, so you can be
certain the loop has stopped when it's called.

Putting all that together, you should have something like this:

    def stop_loop():
        input('Press <enter> to stop')
        loop.call_soon_threadsafe(loop.stop)

    loop = asyncio.get_event_loop()
    threading.Thread(target=stop_loop).start()
    loop.run_forever()
    loop.close()  # optional