Inheritance Question

[Gabriel Genellina]

At Friday 10/11/2006 21:13, Jackson wrote:

>I've got an inheritance question and was hoping brighter minds could
>guide me.  I am in the strange situation where some of the methods in a
>subclass are actually more general than methods in a superclass.  What
>is the preferred way to handle such situations.  My original thought was
>to do something like this:
>
>class AA(object):
>         def general_method(): pass
>
>class A(AA):
>    # redefine general_method() to call a
>    # restricted version of AA.general_method()
>
>class B(A,AA):
>    # redefine general_method() to call AA.general_method()
>
>This seems ugly to me, and I am wondering if there is a better method.
>So any suggestions would be appreciated.

(Note that even using your design, B doesn't have to inherit from 
both A and AA, just inheriting from A is enough to be able to  call 
AA.general_method)

It's hard to tell in this abstract terms, but maybe you should 
consider whether really B "is an" A, and A "is an" AA.
Other kind of relationships are possible, like delegation ("behaves 
like") or the strategy pattern ("now behaves like...", at runtime).

>For a more "concrete" example:

Ahhhh! I didn't notice this when I read your post.

>Suppose all the animals in the world have only 1 or 2 legs.

I would not consider walk a method of Legs, but of Animal. An Animal 
"has" Legs; it may have OneLeg or TwoLegs. An Animal walks "using" its legs.

class Animal:
     def __init__(self, number_of_legs):
         # an animal has legs
         self.legs = CreateLegs(self, number_of_legs)

     def walk(self):
         # an animal uses its legs to walk
         self.legs.walk()

class Legs:
     def walk():
         raise NotImplementedError # an abstract Legs doesn't have how to walk

class OneLeg(Legs): # a monopod? like in Plinius? 
"http://en.wikipedia.org/wiki/Monopod_(creature)"
     def walk():
          # implement walking with one leg
          print "Look ma, just one leg!"

class TwoLegs(Legs):
     def walk():
          # implement walking with two legs
          print "Left, rigth, left, right..."

def CreateLegs(animal, number_of_legs):
     # legs might depend on animal too
     if number_of_legs==1: return OneLeg()
     elif number_of_legs==2: return TwoLegs()
     raise ValueError, "Invalid number of legs: %d" % number_of_legs

If walking in general, have some common structure, you can put the 
"sketch" on Legs and let the derived classes "fill the gaps". This is 
known as "Template Method Pattern" - look for it.


-- 
Gabriel Genellina
Softlab SRL 

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