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[Python-ideas] PEP 3156 EventLoop: hide details of iterations and idleness?

Guido van Rossum guido at python.org
Tue Jan 22 20:19:04 CET 2013 
On Tue, Jan 22, 2013 at 8:16 AM, Geert Jansen <geertj at gmail.com> wrote:
> On Tue, Jan 22, 2013 at 4:31 PM, Ben Darnell <ben at bendarnell.com> wrote:
>
>> I don't understand what you mean by "events will always originate from IO"
>> (I don't know anything about libdbus).
>
> What I meant is that if there is something to dispatch, then this is
> due to an inbound IO (or a timeout for that matter). Due either event,
> the loop will advance by one tick, and hit my call_every_iteration()
> handler where I dispatch.
>
>> If the events are coming from IO
>> that causes an event loop iteration, it must be from some tulip callback.
>> Why can't that callback be responsible for scheduling any further
>> dispatching that may be needed?
>
> Well your original question was why not call_repeatedly() instead of
> call_every_iteration(). I tried to answer that for my use case.
>
> Indeed, call_soon() could be used to schedule a dispatch every time
> when an IO is received. However, I preferred to have a fixed callback
> that I do not need to allocate and register every time, for
> efficiency.
>
>>> If Tornado doesn't have infrastructure for call_every_iteration() you
>>> could emulate it with a function that re-reschedules itself using
>>> call_soon() just before calling the callback. (See my first point
>>> about when call_soon() callbacks are scheduled.)
>>
>>
>> No, because call_soon (and call_later(0)) cause the event loop to use a
>> timeout of zero on its next poll call, so a function that reschedules itself
>> with call_soon will be a busy loop.  There is no good way to emulate
>> call_every_iteration from the other methods; you'll either busy loop with
>> call_soon or use a fixed timeout.  If you need it it's an easy thing to
>> offer, but since neither tornado nor twisted have such a method I'm
>> questioning the need.
>
> Yes, you're right. I was confusing things with libuv and libev. I may
> have actually implemented call_soon() the wrong way there :)
>
> Maybe I am abusing call_every_iteration() when I use it for
> dispatching. If you look at the libuv and libev documentation, then
> they say that their call_every_iteration() equivalents (Prepare and
> Check) are for integrating with external event loops. So maybe that is
> the use case. However, I've not looked into this in any detail.
>
> If Tornado and Twisted cannot implement call_every_iteration(), then I
> think that is a good reason to remove it.

Ok, I'll kill call_every_iteration(). I'll wait for more discussion on
run_once() and run()'s until-idle behavior.

-- 
--Guido van Rossum (python.org/~guido)
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