[Numpy-discussion] ndarray and lazy evaluation (was: Proposed Rodmap Overview)

Olivier Delalleau shish@keba...
Mon Feb 20 12:07:13 CST 2012


Never mind. The link Francesc posted answered my question :)

-=- Olivier

Le 20 février 2012 12:54, Olivier Delalleau <delallea@iro.umontreal.ca> a
écrit :

> Le 20 février 2012 12:46, Dag Sverre Seljebotn <d.s.seljebotn@astro.uio.no
> > a écrit :
>
> On 02/20/2012 09:24 AM, Olivier Delalleau wrote:
>> > Hi Dag,
>> >
>> > Would you mind elaborating a bit on that example you mentioned at the
>> > end of your email? I don't quite understand what behavior you would like
>> > to achieve
>>
>> Sure, see below. I think we should continue discussion on numpy-discuss.
>>
>> I wrote:
>>
>> > You need at least a slightly different Python API to get anywhere, so
>> > numexpr/Theano is the right place to work on an implementation of this
>> > idea. Of course it would be nice if numexpr/Theano offered something as
>> > convenient as
>> >
>> > with lazy:
>> >      arr = A + B + C # with all of these NumPy arrays
>> > # compute upon exiting...
>>
>> More information:
>>
>> The disadvantage today of using Theano (or numexpr) is that they require
>> using a different API, so that one has to learn and use Theano "from the
>> ground up", rather than just slap it on in an optimization phase.
>>
>> The alternative would require extensive changes to NumPy, so I guess
>> Theano authors or Francesc would need to push for this.
>>
>> The alternative would be (with A, B, C ndarray instances):
>>
>> with theano.lazy:
>>     arr = A + B + C
>>
>> On __enter__, the context manager would hook into NumPy to override it's
>> arithmetic operators. Then it would build a Theano symbolic tree instead
>> of performing computations right away.
>>
>> In addition to providing support for overriding arithmetic operators,
>> slicing etc., it would be necesarry for "arr" to be an ndarray instance
>> which is "not yet computed" (data-pointer set to NULL, and store a
>> compute-me callback and some context information).
>>
>> Finally, the __exit__ would trigger computation. For other operations
>> which need the data pointer (e.g., single element lookup) one could
>> either raise an exception or trigger computation.
>>
>> This is just a rough sketch. It is not difficult "in principle", but of
>> course there's really a massive amount of work involved to work support
>> for this into the NumPy APIs.
>>
>> Probably, we're talking a NumPy 3.0 thing, after the current round of
>> refactorings have settled...
>>
>> Please: Before discussing this further one should figure out if there's
>> manpower available for it; no sense in hashing out a castle in the sky
>> in details. Also it would be better to talk in person about this if
>> possible (I'm in Berkeley now and will attend PyData and PyCon).
>>
>> Dag
>>
>
> Thanks for the additional details.
>
> I feel like this must be a stupid question, but I have to ask: what is the
> point of being lazy here, since the computation is performed on exit anyway?
>
> -=- Olivier
>
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