[Numpy-discussion] matrices and __radd__

[Keith Goodman]

On Wed, May 21, 2008 at 4:07 PM, Robert Kern <robert.kern at gmail.com> wrote:
> On Wed, May 21, 2008 at 5:28 PM, Keith Goodman <kwgoodman at gmail.com> wrote:
>> I have a class that stores some of its data in a matrix. I can't
>> figure out how to do right adds with a matrix. Here's a toy example:
>>
>> class Myclass(object):
>>
>>    def __init__(self, x, a):
>>        self.x = x  # numpy matrix
>>        self.a = a  # some attribute, say, an integer
>>
>>    def __add__(self, other):
>>        # Assume other is a numpy matrix
>>        return Myclass(self.x + other, self.a += 1)
>>
>>    def __radd__(self, other):
>>        print other
>>
>>>> from myclass import Myclass
>>>> import numpy.matlib as mp
>>>> m = Myclass(mp.zeros((2,2)), 1)
>>>> x = mp.asmatrix(range(4)).reshape(2,2)
>>>> radd = x + m
>> 0
>> 1
>> 2
>> 3
>>
>> The matrix.__add__ sends one element at a time. That sounds slow.
>
> Well, what's actually going on here is this: ndarray.__add__() looks
> at m and decides that it doesn't look like anything it can make an
> array from. However, it does have an __add__() method, so it assumes
> that it is intended to be a scalar. It uses broadcasting to treat it
> as if it were an object array of the shape of x with each element
> identical. Then it adds together the two arrays element-wise. Each
> element-wise addition triggers the MyClass.__radd__() call.

Oh, broadcasting. OK that makes sense.

>> Do I
>> have to grab the corresponding element of self.x and add it to the
>> element passed in by matrix.__add__? Or is there a better way?
>
> There probably is, but not being familiar with your actual use case,
> I'm not sure what it would be.

From

http://projects.scipy.org/pipermail/numpy-discussion/2006-December/025075.html

I see that the trick is to add

__array_priority__ = 10

to my class.

class Myclass(object):

    __array_priority__ = 10

    def __init__(self, x, a):
        self.x = x  # numpy matrix
        self.a = a  # some attribute, say, an integer

    def __add__(self, other):
        # Assume other is a numpy matrix
        return Myclass(self.x + other, 2*self.a)

    __radd__ = __add__

>> from myclass import Myclass
>> import numpy.matlib as mp
>> m = Myclass(mp.zeros((2,2)), 1)
>> x = mp.asmatrix(range(4)).reshape(2,2)
>> radd = x + m
>> radd.a
   2
>> radd.x

matrix([[ 0.,  1.],
        [ 2.,  3.]])