New submission from Corbin Simpson:
Hi there!
While assisting somebody on #python (where manners go to die), I was consulting
asyncio's documentation. Given my unfamiliarity with asyncio, I was surprised
to read BaseEventLoop.call_soon()'s documented behavior: Arrange for a
callback to be called as soon as possible. [1]
The question in mind is whether call_soon() waits for control to return to the
event loop before calling the first callback in the queue. After all, given the
documented behavior, it would not be unreasonable for call_soon() to always
call callbacks immediately if no other callbacks were enqueued. I can imagine
scenarios where a previous queued callback from e.g. call_soon_threadsafe()
creates a callback that causes queueing behavior, but otherwise I would expect
as soon as possible to mean immediately.
It's true that in other event loops, this kind of callback registration waits
for control to return to the event loop before running callbacks. However, many
event loops that I've worked with indicate that call_later() is the correct API
for adding callbacks that respect the event loop. In fact, call_later(0,
callback, args) looks quite a bit like the familiar Twisted callLater(0,
callback, args) pattern!
Additionally, there is no indication as to whether call_later() and call_soon()
use the same queue.
I'd like clarification on these points in the Python 3 documentation, please.
I'd like to know whether call_soon() can be immediate, and whether call_soon()
preempts or interleaves with call_later().
(Yes, I know that Twisted's documentation is missing this too [2] [3]. I'll get
to that soon enough.)
Thanks!
~ C.
[1]:
https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.BaseEventLoop.call_soon
[2]: http://twistedmatrix.com/documents/current/core/howto/time.html
[3]:
http://twistedmatrix.com/documents/current/api/twisted.internet.interfaces.IReactorTime.html#callLater
--
assignee: docs@python
components: Documentation, asyncio
messages: 231180
nosy: Corbin.Simpson, docs@python, gvanrossum, haypo, yselivanov
priority: normal
severity: normal
status: open
title: asyncio: call_soon() documentation unclear on timing
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http://bugs.python.org/issue22875
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