[Numpy-discussion] subclassing matrix

[Timothy Hochberg]

On Jan 11, 2008 9:59 PM, Basilisk96 <basilisk96 at gmail.com> wrote:

> On Jan 11, 2008, Colin J. Williams wrote:
>
> > You make a good case that it's good not
> > to need to ponder what sort of
> > vector you are dealing with.
> >
> > My guess is that the answer to your
> > question is "no" but I would need to
> > play with your code to see that.  My
> > feeling is that, at the bottom of
> > the __new__ module, the returned object
> > should be an instance of the
> > Vector class.
> >
> > It's been a while since I've worked with
> > numpy and so I'll look at it
> > and hope that someone gives you a
> > definitive answer before I sort it out.
> >
> > Colin W.
>
> Well, let's say that I get rid of that class promotion line. When the
> input object to the constructor is a string or a tuple
> such as Vector('1 2 3') or Vector([1,2,3]), then the returned object
> is always an instance of Vector. However, when the input object is a
> numpy.matrix instance, the returned object remains a numpy.matrix
> instance! So by doing that little hack, I promote it to Vector.
>
> BUT...
>
> It seems that I have solved only half of my problem here. The other
> half rears its ugly head when I perform operations between instances
> of numpy.matrix and Vector. The result ends up returning a matrix,
> which is bad because it has no knowledge of any custom Vector
> attributes. Here's a simple case:
>
>  u = Vector('1 2 3')  #Vector instance
>  P = numpy.mat(numpy.eye(3))   #matrix instance
>  u_new = P*u   #matrix instance, not desirable!
>  u_new_as_Vector = Vector(P*u)  #Vector instance
>
> I'd rather not have to remember to re-instantiate the result in client
> code. I think I understand why this is happening - the code in
> numpy.core.defmatrix.matrix.__mul__ goes like this:
>
>    def __mul__(self, other):
>        if isinstance(other,(N.ndarray, list, tuple)) :
>            # This promotes 1-D vectors to row vectors
>            return N.dot(self, asmatrix(other))
>        if N.isscalar(other) or not hasattr(other, '__rmul__') :
>            return N.dot(self, other)
>        return NotImplemented
>
> It passes the first condition: isinstance(other,(N.ndarray)) is true;
> and so the return value becomes a matrix.
>
> Bummer.
> Do I also need to override a few overloaded methods like __mul__,
> __rmul__, etc. to make this work?



I believe that you need to look at __array_finalize__ and __array_priority__
(and there may be one other thing as well, I can't remember; it's late).
Search for __array_finalize__ and that will probably help get you started.



-- 
.  __
.   |-\
.
.  tim.hochberg at ieee.org
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