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