[Numpy-discussion] asanyarray vs. asarray

[Marten van Kerkwijk]

The substitution principle is interesting (and, being trained as an
astronomer, not a computer scientist, I had not heard of it before). I
think matrix is indeed obviously wrong here (with indexing being more
annoying, but multiplication being a good example as well).

Perhaps more interesting as an example to consider is MaskedArray, which is
much closer to a sensible subclass, though different from Quantity in that
what is masked can itself be an ndarray subclass. In a sense, it is more of
a container class, in which the operations are done on what is inside it,
with some care taken about which elements are fixed. This becomes quite
clear when one thinks of implementing __array_ufunc__ or
__array_function__: for Quantity, calling super after dealing with the
units is very logical, for MaskedArray, it makes more sense to call the
(universal) function again on the contents [1].

For this particular class, if reimplemented, it might make most sense as a
"mixin" since its attributes depend both on the masked class (.mask, etc.)
and on what is being masked (say, .unit for a quantity). Thus, the final
class might be an auto-generated new class (e.g.,
MaskedQuantity(MaskedArray, Quantity)). We have just added a new
Distribution class to astropy which is based on this idea [2] (since this
uses casting from structured dtypes which hold the samples to real arrays
on which functions are evaluated, this probably could be done just as well
or better with more flexible dtypes, but we have to deal with what's
available in the real world, not the ideal one...).

-- Marten

[1]
http://www.numpy.org/neps/nep-0013-ufunc-overrides.html#subclass-hierarchies
[2] https://github.com/astropy/astropy/pull/6945
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