[Python-ideas] yield from multiple iterables (was Re: The async API of the future: yield-from)

Christian Tismer tismer at stackless.com
Sat Oct 20 00:31:15 CEST 2012

Hi Guido, Marc, all,

this is a veery promising result, telling
me that the big Oh can in fact be
neglected in real applications. 20 for
twi is good!

I will of course do an analysis and find
the parameters of the quadratic, but my
concern is pretty much tamed. 

For me that means there will soon be
a library that contains real generators
and more building blocks. 

I think using those would simplify the
design of the async API quite a lot. 

I suggest to regard current generator
constructs as low-level helpers for
Implementing the real concurrency
building blocks. 

Instead of using the existing re.compile("yield (from)?") pattern, I think we can abstract
from this now and think in terms of
higher level constructs. 

Let's assume generators and coroutines,
and model concurrency from that. I
believe this unwinds the brains and
clarifies things a lot. 

I will provide sone classes for that at
the pycon.de sprint, unless somebody
implements it earlier (please don't).

This email was written in non-linear
order, so please ignore logic inversions. 

Cheers - chris

Sent from my Ei4Steve

On Oct 19, 2012, at 23:31, Guido van Rossum <guido at python.org> wrote:

> On Fri, Oct 19, 2012 at 2:15 PM, Mark Shannon <mark at hotpy.org> wrote:
>> On 19/10/12 13:55, Christian Tismer wrote:
>>> Hi Nick,
>>> On 16.10.12 03:49, Nick Coghlan wrote:
>>>> On Tue, Oct 16, 2012 at 10:44 AM, Greg Ewing
>>>> <greg.ewing at canterbury.ac.nz> wrote:
>>>>> My original implementation of yield-from actually *did* avoid
>>>>> this, by keeping a C-level pointer chain of yielding-from frames.
>>>>> But that part was ripped out at the last minute when someone
>>>>> discovered that it had a detrimental effect on tracebacks.
>>>>> There are probably other ways the traceback problem could be
>>>>> fixed, so maybe we will get this optimisation back one day.
>>>> Ah, I thought I remembered something along those lines. IIRC, it was a
>>>> bug report on one of the alphas that prompted us to change it.
>>> I was curious and searched quite a lot.
>>> It was v3.3.0a1 from 2012-03-15 as a reaction to #14230 and #14220
>>> from Marc Shannon, patched by Benjamin.
>>> Now I found the original implementation. That looks very much
>>> as I'm thinking it should be.
>>> Quite a dramatic change which works well, but really seems to remove
>>> what I would call "now I can emulate most of Stackless" efficiently.
>>> Maybe I should just try to think it would be implemented as before,
>>> build an abstraction and just use it for now.
>>> I will spend my time at PyCon de for sprinting on "yield from".
>> The current implementation may not be much slower than Greg's original
>> version. One of the main costs of making a call is the creation of a new
>> frame. But calling into a generator does not need a new frame, so the cost
>> will be reduced.
>> Unless anyone has evidence to the contrary :)
>> Rather than increasing the performance of this special case, I would suggest
>> that improving the performance of calls & returns in general would be a more
>> worthwhile goal.
>> Calls and returns ought to be cheap.
> I did a basic timing test using a simple recursive function and a
> recursive PEP-380 coroutine computing the same value (see attachment).
> The coroutine version is a little over twice as slow as the function
> version. I find that acceptable. This went 20 deep, making 2 recursive
> calls at each level (except at the deepest level).
> Output on my MacBook Pro:
> plain 2097151 0.5880069732666016
> coro. 2097151 1.2958409786224365
> This was a Python 3.3 built a few days ago from the 3.3 branch.
> -- 
> --Guido van Rossum (python.org/~guido)
> <p3time.py>
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