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When is a subclass not right?

Gabriel Genellina gagsl-py at yahoo.com.ar
Thu Aug 24 16:21:21 EDT 2006 
At Thursday 24/8/2006 16:23, Chaz Ginger wrote:

>I was writing some code that used someone else class as a subclass. He
>wrote me to tell me that using his class as a subclass was incorrect. I
>am wondering under what conditions, if ever, does a class using a
>subclass not work.
>
>class B1(A);
>   def __init__(self,a1,a2) :
>         self.c = a1
>         A.__init__(self,ag)
>
>class B2:
>   def __init__(self,a1,a2):
>         self.c = a1
>         self.t = A(a2)
>
>   def bar(self) :
>         self.t.bar()
>
>Other than the obvious difference of B2 having an attribute 't', I can't
>see any other obvious differences. Is there something I am missing?

Look any OO book for the difference between 'inheritance' and 
'delegation'. In short, you should inherit when B 'is an' A (a Car is 
a Vehicle), and delegate/compose in other cases (a Car has an Engine; 
or more precisely, a Car instance has an Engine instance).


Gabriel Genellina
Softlab SRL 


	
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