asyncio - how to stop loop?
Frank Millman
frank at chagford.com
Thu Jun 12 04:00:44 EDT 2014
"Ian Kelly" <ian.g.kelly at gmail.com> wrote in message
news:CALwzidnv07Wba9WJ=Nuc0_v4mvudYAXWh6BgjvW0o1hF3ooWkg at mail.gmail.com...
> 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
Thanks very much for the very clear explanation.
Your solution works perfectly.
Much appreciated.
Frank
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