[Cython] buffer shape incompatible with memoryview shape

[Dag Sverre Seljebotn]

On 06/21/2012 02:59 PM, Stefan Behnel wrote:
> Dag Sverre Seljebotn, 21.06.2012 14:05:
>> On 06/21/2012 01:36 PM, Stefan Behnel wrote:
>>>> On 06/21/2012 10:59 AM, Stefan Behnel wrote:
>>>>> I find this worth fixing for 0.17:
>>>>>
>>>>> http://trac.cython.org/cython_trac/ticket/780
>>>>
>>> I ran into this when I gave a Cython+NumPy course and this was the first
>>> thing that the attendants tried when I asked them to validate that two
>>> input arrays have the same size before adding them. It's the one obvious
>>> way to do it, and it fails miserably. I think it should be fixed, and I
>>> think it should be fixed soon because it feels really low-level and
>>> complicated, especially to new users.
>>
>> Can you clarify a bit -- did you give this course using np.ndarray[double,
>> ndim=2], or double[:, :]? They're really very separate under the hood and
>> the fix is different.
>>
>> Or, did you actually use object[double, ndim=2] like in the bug report?
>> (Did me and Mark get around to propose deprecating this one on the list?)
>
> IIRC, we used object[double, ndim=2] for both and I also tried it with a
> memory view as in the bug report. I thought that using "object" was the
> preferred way to do it? At least, it doesn't restrict the type of the
> buffer exporter, which I consider a good thing.

That's a very theoretical argument as NumPy arrays are in practice the 
only exporter.

I always teach np.ndarray[double...]. I've never told anyone about 
object[...], I don't think it's in much use. For starters it's going to 
be horribly inefficient unless you also add "mode='strided'" within the 
brackets.

My proposal (and Mark's I think) is:

Since the memoryviews will neatly cover the general exporter case, and 
since the [] syntax is much overloaded already (used for C++ templates 
too), we should deprecate object[...] no matter what else happens.

Depending on what's decided for np.ndarray[...], we have:

Case A): Deprecate both np.ndarray[...] and object[...]

Case B): Only deprecate object[...], keep np.ndarray[...] (e.g., through 
a decorator used in numpy.pxd on the ndarray type). So rather than 
having a trailing [] mean buffers unless it means something else (like 
C++ templates), we instead make np.ndarray a "template", through special 
compiler support.


Dag