[Numpy-discussion] Subclassing ndarray with concatenate

Sebastian Berg sebastian@sipsolutions....
Wed Jan 30 04:20:39 CST 2013

On Wed, 2013-01-30 at 10:24 +0100, Todd wrote:
> On Tue, Jan 22, 2013 at 1:44 PM, Sebastian Berg
> <sebastian@sipsolutions.net> wrote:
>         Hey,
>         On Tue, 2013-01-22 at 10:21 +0100, Todd wrote:
>         > The main exception I have found is concatenate (and
>         hstack/vstack,
>         > which just wrap concatenate).  In this case,
>         __array_finalize__ is
>         > passed an array that has already been stripped of the
>         additional
>         > attributes, and I don't see a way to recover this
>         information.
>         >
>         There are quite a few functions that simply do not preserve
>         subclasses
>         (though I think more could/should call `__array_wrap__`
>         probably, even
>         if the documentation may say that it is about ufuncs, there
>         are some
>         example of this already).
>         `np.concatenate` is one of these. It always returns a base
>         array. In any
>         case it gets a bit difficult if you have multiple input arrays
>         (which
>         may not matter for you).
> I don't think this is right.  I tried it and it doesn't return a base
> array, it returns an instance of the original array subclass.

Yes you are right it preserves type, I was fooled by
`__array_priority__` being 0 as default, thought it defaulted to more
then 0 (for ufuncs everything beats arrays, not sure if it really
should) but so I missed.

In any case, yes, it calls __array_finalize__, but as you noticed, it
calls it without the original array. Now it would be very easy and
harmless to change that, however I am not sure if giving only the parent
array is very useful (ie. you only get the one with highest array

Another way to get around it would be maybe to call __array_wrap__ like
ufuncs do (with a context, so you get all inputs, but then the non-array
axis argument may not be reasonably placed into the context).

In any case, if you think it would be helpful to at least get the single
parent array, that would be a very simple change, but I feel the whole
subclassing could use a bit thinking and quite a bit of work probably,
since I am not quite convinced that calling __array_wrap__ with a
complicated context from as many functions as possible is the right
approach for allowing more complex subclasses.

>         > In my particular case at least, there are clear ways to
>         handle corner
>         > cases (like being passed a class that lacks these
>         attributes), so in
>         > principle there no problem handling concatenate in a general
>         way,
>         > assuming I can get access to the attributes.
>         >
>         >
>         > So is there any way to subclass ndarray in such a way that
>         concatenate
>         > can be handled properly?
>         >
>         Quite simply, no. If you compare masked arrays, they also
>         provide their
>         own concatenate for this reason.
>         I hope that helps a bit...
> Is this something that should be available?  For instance a method
> that provides both the new array and the arrays that were used to
> construct it.  This would seem to be an extremely common use-case for
> array subclasses, so letting them gracefully handle this would seem to
> be very important.
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