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Date: Sat, 23 Nov 2013 21:30:29 +0100
From: "Tobias M." <tm@tobix.eu>
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To: python-list@python.org
Subject: Re: Periodic execution with asyncio
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Thanks a lot for your helpful posts, Terry!

On 11/23/2013 01:00 AM, Terry Reedy wrote:
> * Make the task function a parameter 'func'.
I actually like subclassing, but yes I know there are some downsides :)
>
> * Rename start to _set to better describe what is does and call it in 
> the _run function to not repeat the handler setting line. (If 'start' 
> has any magic meaning, I am not aware of it. I am aware that there 
> could be situations in which one would want a public method, to be 
> called from other handlers. But that would be a different class ;-)
>
> * Comment out the stop method because it is useless in the current 
> usage. I believe handler.cancel is only relevant after the handler is 
> set and before it is fired. This can only be done from another handler.
You are right, the handler setting line should not be repeated.

I choose start() and stop() for controlling the task because this is an 
interface I would instantly understand as a user of this class.
By the way: My whole motivation is, that I want to implement such a task 
class in a network library I am currently writing.


On 11/23/2013 02:33 AM, Terry Reedy wrote:
> I was initially baffled also until I managed to assemble all the 
> needed pieces.
>
> import asyncio
>
> def f():
>     print('Hello World')
>
> @asyncio.coroutine
> def g(func, interval):
>     while True:
>         yield from asyncio.sleep(interval)
>         func()
>
> loop = asyncio.get_event_loop()
>
> try:
>     loop.run_until_complete(asyncio.Task(g(f, 2)))
> except KeyboardInterrupt:
>     print('Loop stopped')
>
Thanks, now I got it. I just couldn't figure out that I have to yield 
from the sleep function.

Now putting this into a PeriodicTask class that provides a similar 
interface like our callback version, I get:


import asyncio

class PeriodicTask2(object):

      def __init__(self, func, interval):
          self.func = func
          self.interval = interval
          self._loop = asyncio.get_event_loop()

      def start(self):
          self.loop.run_until_complete(asyncio.Task(self._run()))

     @asyncio.coroutine
      def _run(self):
          while True:
                 yield from asyncio.sleep(self.interval)
                 self.func()


I don't know if I misunderstood anything, but as a user of this class I 
am not able to run two task simultaneously because start() will block. 
In the callback version I could instanciate two PeriodicTasks and run 
them both at the same time (after a call to loop.run_forever()), which 
is actually what I wanted to achieve with this class.