[Numpy-discussion] deprecate numpy.matrix

[Pauli Virtanen]

Alan G Isaac <alan.isaac <at> gmail.com> writes:
[clip]
> Here, quacking is behaving like an ndarray (in your view,
> as I understand it) when asked.  But how do we ask?
> Your view (if I understand) is we ask via the operations
> supported by ndarrays.  But maybe that is the wrong way
> for the library to ask this question.

It is not a good thing that there is no well defined
"domain specific language" for matrix algebra in Python.

Rather, some code is written with one convention and other
code with a different convention. The conventions disagree
on how to express basic operations, such as matrix
multiplication.

Moreover, the ndarray is also lacking some useful things, as
you point out. But I think the right solution would be to stuff
the required additions into ndarray, rather than retaining the
otherwise incompatible np.matrix as a crutch.

> If so, then scipy libraries could ask an object
> to behave like an an ndarray by calling, e.g.,
> __asarray__ on it. It becomes the responsibility
> of the object to return something appropriate
> when __asarray__ is called. Objects that know how to do
> this will provide __asarray__ and respond
> appropriately.

Another way to achieve similar thing as your suggestion is to add
a coercion function in the vein of scipy.sparse.aslinearoperator.
It could deal with known-failure cases (np.matrix, scipy.sparse matrices)
and for the rest just assume the object satisfies the ndarray API
and pass them through.

-- 
Pauli Virtanen