A generic question: idiom for a paramterized base class?

[Ian Bicking]

On Tue, 2002-07-30 at 16:25, Blair Hall wrote:
> ############ example.py ###########
> class GenericBase(object):
>     def __iadd__(self,other):
>         return TAdd(self,other) # Problem!!
>     def __add__(self,r):
>         return GenericBase.__iadd__(self,r)
>     def __radd__(self,l):
>         return GenericBase.__iadd__(l,self)
> 
> # One type of TAdd:
> class TAdd(object,GenericBase):
>     def __init__(self,l,r):
>         print "TAdd instance"
> 
> class TNode(object,GenericBase):
>     def __init__(self):
>         print "TNode instance"
> ############################
> 
> >>> import example
> >>> x = example.TNode()
> TNode instance
> >>> y = example.TNode()
> TNode instance
> >>> t = x + y
> TAdd instance
> 
> However, the first class, GenericBase, is actually one that I need to
> reuse in a number
> of different situations, each with different TAdd definitions!

I'm unclear about what you want to do with this... but maybe something
like this would work:

class GenericBase(object):
    def __init__(self, addClass):
        self._addClass = addClass
    def __iadd__(self, other):
        return self._addClass(self, other)

class GenericBaseBuilder:
    def __init__(self, addClass):
        self._addClass = addClass
    def __call__(self):
        return GenericBase(self._addClass)

# then in example.py:
class TAdd: pass 

GenericBase = GenericBaseBuilder(TAdd)


That might do what you want.  To me, though, the whole thing seems vague
and maybe there's another approach that avoids some of these
complexities.

Cheers,
  Ian