[EMAIL PROTECTED] wrote: > # This is what I have in mind: > > class Item(object): > def __add__(self, other): > return Add(self, other) > > class Add(Item): > def __init__(self, a, b): > self.a = a > self.b = b > > a = Item() > b = Item() > > c = a+b > > # Now, I am going absolutely crazy with this idea > # and using it in a big way. So I'm looking at > # automating the process. As a first step, > # I thought maybe this would work: > > class Item(object): > pass > > class Add(Item): > def __init__(self, a, b=None): > print self, a, b > self.a = a > self.b = b > > Item.__add__ = Add > > x = Item() > y = Item() > > print x, y > > c = x+y > > # This time, the Add constructor gets only the first two arguments: > "self" and "y". > # So, what happened to "x" ? Is this some kind of property voodoo going > on ? > > # Simon.
Look at the signature for __add__: __add__(self, other) Now look at the signature for __init__: __init__(self, a, b=None) Now look at the output of your script: |>> <__main__.Item object at 0xb6faa4ac> <__main__.Item object at 0xb6faa3ec> <__main__.Add object at 0xb6faa5ec> <__main__.Item object at 0xb6faa3ec> None When python calls x.__add__ as a result of "x+y" it only passes it y as 'other', or, in this case, as 'a'. The 'self' arg gets filled in with a brand new Add instance, because __add__ (rather than being a method of Item) is really the class Add. (b will never be filled in using the Add.__init__ method this way.) Consider: |>> x <__main__.Item object at 0xb6f10f0c> |>> x.__add__ <class '__main__.Add'> |>> Badd = x.__add__ |>> Badd <class '__main__.Add'> |>> b = Badd(23, 18) <__main__.Add object at 0xb6f10e8c> 23 18 |>> b <__main__.Add object at 0xb6f10e8c> Make more sense? :-) "Item.__add__ = Add" is a very strange thing to do, I'm not surprised it didn't work. HTH, ~Simon -- http://mail.python.org/mailman/listinfo/python-list
