[Numpy-discussion] asarray/anyarray; matrix/subclass

Matti Picus matti.picus at gmail.com
Sat Nov 10 11:02:23 EST 2018

On 9/11/18 5:09 pm, Nathaniel Smith wrote:
> On Fri, Nov 9, 2018 at 4:59 PM, Stephan Hoyer <shoyer at gmail.com> wrote:
>> On Fri, Nov 9, 2018 at 6:46 PM Nathaniel Smith <njs at pobox.com> wrote:
>>> But matrix isn't the only problem with asanyarray. np.ma also violates
>>> Liskov. No doubt there are other problematic ndarray subclasses out
>>> there too...
>>> Please forgive my ignorance (I don't really use mask arrays), but how
>>> specifically do masked arrays violate Liskov? In most cases shouldn't they
>>> work the same as base numpy arrays, except with operations keeping track of
>>> masks?
> Since many operations silently skip over masked values, the
> computation semantics are different. For example, in a regular array,
> sum()/size() == mean(), but with a masked array these are totally
> different operations. So if you have code that was written for regular
> arrays, but pass in a masked array, there's a solid chance that it
> will silently return nonsensical results.
> (This is why it's better for NAs to propagate by default.)
> -n

Echos of the discussions in neps 12, 24, 25, 26. http://www.numpy.org/neps


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