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

Nathaniel Smith njs at pobox.com
Fri Nov 9 20:09:04 EST 2018

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.)


Nathaniel J. Smith -- https://vorpus.org

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