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

[Geert Jansen]

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.

Regards,
Geert