[Numpy-discussion] deprecate numpy.matrix

[Alan G Isaac]

On 2/9/2014 5:55 PM, alex wrote:
> I'm working on the same kinds of problems in scipy development
> (functions involving sparse matrices and abstract linear operators)


And how is numpy's matrix object getting in your way?
Your initial post simply treated the desirability of
deprecation as a given and did not lay out reasons.
A strong reason would be e.g. if the matrix object
is creating a serious maintenance headache.  Eliminating
this should be a big enough gain to offset any lost interest
in numpy from users of Matlab, GAUSS, IDL etc. from the
disappearance of a user-friendly notation.

I accept that a numpy matrix has some warts.  In the past,
I've proposed changes to address these.  E.g.,
https://www.mail-archive.com/numpy-discussion@scipy.org/msg06780.html
However these went nowhere, so effectively the status quo was
defended.  I can live with that.

A bit of the notational advantage of the `matrix` object was undercut
by the addition of the `dot` method to arrays. If `matrix` is deprecated,
I would hope that a matrix-power method would be added.  (One that works
correctly with boolean arrays and has a short name.)  I ideally an inverse
method would be added as well (with a short name).  I think adding the
hermitian transpose as `.H()` already has some support, but I forget its current
status.

Right now, to give a simple example, students can write a simple projection
matrix as `X * (X.T * X).I * X.T` instead of `X.dot(la.inv(X.T.dot(X))).dot(X.T)`.
The advantage is obvious and even bigger with more complex expressions.
If we were to get `.I` for matrix inverse of an array (which I expect to be
vociferously resisted) it would be `X.dot(X.T.dot(X).I).dot(X.T)` which
at the moment I'm inclined to see as acceptable for teaching. (Not sure.)

Just to forestall the usual "just start them with arrays, eventually they'll
be grateful" reply, I would want to hear that suggestion only from someone
who has used it successfully with undergraduates in the social sciences.

Alan Isaac