[Numpy-discussion] Unexpected output using numpy.ndarray and __radd__

Mark Hoffmann Mark.Hoffmann at dk.manbw.com
Mon Dec 18 01:30:20 CST 2006


> Hi,
> 
> 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
> 
> # ---------- OUTPUT ----------
> #
> # addprob $ addprob.py
> # __add__ :  Cyclehist object: valuearray = [  1.  -21.2   3.2] [-1.
> 2.2 -2.2]
> # [  0. -19.   1.]
> # __radd__ :  Cyclehist object: valuearray = [  1.  -21.2   3.2] -1.0
> # __radd__ :  Cyclehist object: valuearray = [  1.  -21.2   3.2] 2.2
> # __radd__ :  Cyclehist object: valuearray = [  1.  -21.2   3.2] -2.2
> # [[  0.  -22.2   2.2] [  3.2 -19.    5.4] [ -1.2 -23.4   1. ]]
> # addprob $
> #
> # ----------------------------
> 
> 
> I expected the output of "c+a" and "a+c" to be identical, however, the
> output of "a+c" gets nested in an elementwise fashion. Can anybody
> explain this? Is it a bug or a feature? I'm using Python 2.4.4c1 and
> numpy 1.0. I tried the programme using an older version of Python and
> numpy and there the result of "c+a" and "a+c" are identical.
> 
> 
> Regards,
> 
> Mark Hoffmann
> 
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