Replacing base class

DH no at sp.am
Wed Mar 3 16:54:12 CET 2004


Alexey Klimkin wrote:
> Hello!
> 
> Is it possible in python to replace base class with another one?
> 
> Assume an example:
> class A:
>   def f(self):
>     print 'A'
> class B(A):
>   def f(self):
>     A.f(self)
>     print 'B'
> class C:
>   def f(self):
>     print 'C'
> 
> I need to make class Bm with the same functionality as B, but 
> derived from C, instead of A. Classes C and A have the same
> interface.
> 
> The call of f() for Bm should output:
> C
> B
> 
> As you see, the main problem is B.f, since it uses f() from base
> class. Any clues?
> 

You might want to give some more details about what you are really 
doing, because there are multiple ways to do this, but the simplest is 
to just replace A.f(self) with:
   super(B,self).f(self)
And make your classes subclass "object" so they are new style classes 
(required for "super" to work).

Another option is to make "B" a mixin class that can work with A or C or 
both:
class B: #mixin
     def f(self):
         for base in self.__class__.__bases__:
             if (base is not B) and hasattr(base,"f"):
                 base.f(self)
         print 'B'
class Test(B,C,A): pass #put mixin class first

But you might also want to create an "interface" class that specifies 
the "f" method, which A & C "implement".
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/164901
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/204349
http://peak.telecommunity.com/PyProtocols.html

Then in B, you could call f() on all bases that implement that interface 
if you liked.



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