[Python-ideas] The async API of the future: yield-from

Yuval Greenfield ubershmekel at gmail.com
Sat Oct 20 10:00:28 CEST 2012

On Thu, Oct 18, 2012 at 9:49 AM, Greg Ewing <greg.ewing at canterbury.ac.nz>wrote:

> I've converted my tutorial on generator-based tasks
> for Python 3.3, tidied it up a bit and posted it here:
> http://www.cosc.canterbury.ac.**nz/greg.ewing/python/tasks/<http://www.cosc.canterbury.ac.nz/greg.ewing/python/tasks/>
> --
> Greg
Thanks for writing this. I've used threads all my life so this
coroutine/yield-from paradigm is hard for me to grok even after reading
this quite a few times.

I can't wrap my head around the block and unblock functions.

block() removes the current task from the ready_list, but is the current
task guaranteed to be my task? If so, then I'd never run again after the
yield in acquire(), that is unless a gracious other player unblocks me.

block() in acquire() is the philosopher or fork avoiding the scheduler?
yield in acquire() is the philosopher relinquishing control or the fork?

I think I finally figured it out after staring at it for long enough. I'm
not sure it makes sense for scheduler functions to store waiting tasks in a
queue owned by the app and invisible from the scheduler. This can
cause *invisible
deadlocks* such as:

schedule(philosopher("Socrates", 8, 3, 1, forks[0], forks[2]), "Socrates")
schedule(philosopher("Euclid", 5, 1, 4, forks[2], forks[0]), "Euclid")

Which may be really hard to debug.

Is there a coroutine strategy for tackling these challenges? Or will I just
get better at overcoming them with practice?

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