[Cython] buffer syntax vs. memory view syntax

Dag Sverre Seljebotn d.s.seljebotn at astro.uio.no
Mon May 7 19:10:42 CEST 2012

On 05/07/2012 07:00 PM, Stefan Behnel wrote:
> mark florisson, 07.05.2012 18:28:
>> On 7 May 2012 17:00, Dag Sverre Seljebotn wrote:
>>> On 05/07/2012 04:16 PM, Stefan Behnel wrote:
>>>> Stefan Behnel, 07.05.2012 15:04:
>>>>> Dag Sverre Seljebotn, 07.05.2012 13:48:
>>>>>> BTW, with the coming of memoryviews, me and Mark talked about just
>>>>>> deprecating the "mytype[...]" meaning buffers, and rather treat it as
>>>>>> np.ndarray, array.array etc. being some sort of "template types". That
>>>>>> is,
>>>>>> we disallow "object[int]" and require some special declarations in the
>>>>>> relevant pxd files.
>>>>> Hmm, yes, it's unfortunate that we have two different types of syntax
>>>>> now,
>>>>> one that declares the item type before the brackets and one that declares
>>>>> it afterwards.
>>>> I actually think this merits some more discussion. Should we consider the
>>>> buffer interface syntax deprecated and focus on the memory view syntax?
>>> I think that's the very-long-term intention. Then again, it may be too early
>>> to really tell yet, we just need to see how the memory views play out in
>>> real life and whether they'll be able to replace np.ndarray[double] among
>>> real users. We don't want to shove things down users throats.
>>> But the use of the trailing-[] syntax needs some cleaning up. Me and Mark
>>> agreed we'd put this proposal forward when we got around to it:
>>>   - Deprecate the "object[double]" form, where [dtype] can be stuck on any
>>> extension type
>>>   - But, do NOT (for the next year at least) deprecate np.ndarray[double],
>>> array.array[double], etc. Basically, there should be a magic flag in
>>> extension type declarations saying "I can be a buffer".
>>> For one thing, that is sort of needed to open up things for templated cdef
>>> classes/fused types cdef classes, if that is ever implemented.
>> Deprecating is definitely a good start.
> Then the first step on that road is to rework the documentation so that it
> pushes users into going for memory views instead of the plain buffer syntax.

-1, premature.


>> I think at least if you only
>> allow two types as buffers it will be at least reasonably clear when
>> one is dealing with fused types or buffers.
>> Basically, I think memoryviews should live up to demands of the users,
>> which would mean there would be no reason to keep the buffer syntax.
>> One thing to do is make memoryviews coerce cheaply back to the
>> original objects if wanted (which is likely). Writting
>> np.asarray(mymemview) is kind of annoying.
> ... and also doesn't do the same thing, I believe.
>> Also, OT (sorry), but I'm kind of worried about the memoryview ABI. If
>> it changes (and I intend to do so), cython modules compiled with
>> different cython versions will become incompatible if they call each
>> other through pxds. Maybe that should be defined as UB...
> Would there be a way to only use the plain buffer interface for cross
> module memory view exchange? That could be an acceptable overhead to pay
> for ABI independence.
> Stefan
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