[Numpy-discussion] What's the difference between calling __mul__ and *?

Toder, Evgeny evgeny.toder@jpmorgan....
Fri Jun 7 11:38:22 CDT 2013


That's how it works in python:

"""
Note: If the right operand's type is a subclass of the left operand's type and that subclass provides the reflected method for the operation, this method will be called before the left operand's non-reflected method. This behavior allows subclasses to override their ancestors' operations.
"""
http://docs.python.org/2/reference/datamodel.html#emulating-numeric-types

Note that matrix is a subclass of ndarray.
Also note that __mul__ can return NotImplemented, in which case again the method of rhs argument will be used.

Eugene

-----Original Message-----
From: numpy-discussion-bounces@scipy.org [mailto:numpy-discussion-bounces@scipy.org] On Behalf Of Will Lee
Sent: Friday, June 07, 2013 12:30 PM
To: Discussion of Numerical Python
Subject: [Numpy-discussion] What's the difference between calling __mul__ and *?

Can somebody tell me why these operations are not the same in numpy?

In [2]: a = numpy.array([1, 2, 3.])

In [4]: matrix = numpy.matrix([[1, 2, 3.], [4, 5, 6], [7, 8, 9]])

In [5]: a.__mul__(matrix)

matrix([[  1.,   4.,   9.],

        [  4.,  10.,  18.],

        [  7.,  16.,  27.]])

In [6]: a * matrix

matrix([[ 30.,  36.,  42.]])


Essentially I'm trying to extend from numpy.ndarray.  From my subclass
__mul__ I'd like to call the parent's __mul__ method.  I ran into
problem when I'm trying to call super(SubArrayClass, self).__mul__()
method when working with a matrix.  I also can't think of a way to use
operator.mul() due to the subclass nature.  Is there any way to make
this work?

Any help is greatly appreciated.

Thanks,

Will
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