Excellent, thank you - it solved the problem! /Mark -----Original Message----- From: numpy-discussion-bounces@scipy.org [mailto:numpy-discussion-bounces@scipy.org] On Behalf Of Tim Hochberg Sent: 18. december 2006 14:06 To: Discussion of Numerical Python Subject: Re: [Numpy-discussion] Unexpected output usingnumpy.ndarray and__radd__ Mark Hoffmann wrote:

I appreciate the answer and the solution suggestion. I see that it is possible to make a work around by subclassing from ndarray. Still, in the "print a+c" statement, I don't understand why a.__add__(c) doesn't return NotImplemented (because ndarray shouldn't recognize the Cyclehist class) and directly call c.__radd__(a) implemented in my Cyclehist class. I tried the exactly same programme using Python 2.4.1 and Scipy 0.3.2 (based on numeric/numarray) and the result of the "print a+c" didn't get nested as I expect.

Regards, Mark

I'm not sure what this is doing -- it looks kind of bizzare -- however, you can fix this case without resorting to subclassing to ndarray. Just toss an '__array_priority__ = 10' up at the top of the class definition and it will use your __methods__ in preference to the ndarrays. I don't have time to look into this further right now unfortunately. -tim

-----Original Message----- From: numpy-discussion-bounces@scipy.org [mailto:numpy-discussion-bounces@scipy.org] On Behalf Of Stefan van der Walt Sent: 18. december 2006 10:36 To: Discussion of Numerical Python Subject: Re: [Numpy-discussion] Unexpected output using numpy.ndarray and__radd__

Hi Mark

On Mon, Dec 18, 2006 at 08:30:20AM +0100, Mark Hoffmann wrote:

...

The following issue has puzzled me for a while. I want to add a numpy.ndarray and an instance of my own class. I define this operation by implementing the methods __add__ and __radd__. My programme (including output) looks like:

#!/usr/local/bin/python

import numpy

class Cyclehist: def __init__(self,vals): self.valuearray = numpy.array(vals)

def __str__(self): return 'Cyclehist object: valuearray = '+str(self.valuearray)

def __add__(self,other): print "__add__ : ",self,other return self.valuearray + other

def __radd__(self,other): print "__radd__ : ",self,other return other + self.valuearray

c = Cyclehist([1.0,-21.2,3.2]) a = numpy.array([-1.0,2.2,-2.2]) print c + a print a + c

In the first instance, c.__add__(a) is called, which works fine. In the second, a.__add__(c) is executed, which is your problem, since you rather want c.__radd__(a) to be executed. A documentation snippets:

"""For instance, to evaluate the expression x-y, where y is an instance of a class that has an __rsub__() method, y.__rsub__(x) is called if x.__sub__(y) returns NotImplemented.

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."""

Since a.__add__ does not return NotImplemented, c.__radd__ is not called where you expect it to be. I am not sure why broadcasting takes place here, maybe someone else on the list can elaborate.

To solve your problem, you may want to look into subclassing ndarrays, as described at http://www.scipy.org/Subclasses.

Cheers Stéfan _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion

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