[Numpy-discussion] Subclassing ndarray with concatenate

Sebastian Berg sebastian@sipsolutions....
Tue Jan 22 09:56:05 CST 2013


On Tue, 2013-01-22 at 13:44 +0100, Sebastian Berg wrote:
> Hey,
> 
> On Tue, 2013-01-22 at 10:21 +0100, Todd wrote:
> > I am trying to create a subclass of ndarray that has additional
> > attributes.  These attributes are maintained with most numpy functions
> > if __array_finalize__ is used.  
> > 
> You can cover a bit more if you also implement `__array_wrap__`, though
> unless you want to do something fancy, that just replaces the
> `__array_finalize__` for the most part. But some (very few) functions
> currently call `__array_wrap__` explicitly.
> 

Actually have to correct myself here. The default __array_wrap__ causes
__array_finalize__ to be called as you would expect, so there is no need
to use it unless you want to do something fancy.

> > 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).
> 
> > 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...
> 
> Regards,
> 
> Sebastian
> 
> > I have been looking extensively online, but have not been able to find
> > a clear answer on how to do this, or if there even is a way.
> > 
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> > http://mail.scipy.org/mailman/listinfo/numpy-discussion
> 
> 
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