lazy arithmetic

[Peter Otten]

pianomaestro at gmail.com 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 ?

Indeed. Read up on descriptors:

http://users.rcn.com/python/download/Descriptor.htm

The following seems to work:

from new import instancemethod

class Add:
    class __metaclass__(type):
        def __get__(self, inst, cls):
            if inst is None:
                return cls
            return instancemethod(cls, inst, cls)
    def __init__(self, a, b=None):
        self.a = a
        self.b = b
    def __str__(self):
        return "%s(%s, %s)" % (self.__class__.__name__, self.a, self.b)

Add.__add__ = Add
print Add(1, 2) + 42

but makes the simple

def __add__(self, other): 
    return Add(self, other)

all the more attractive.

Peter