When is a subclass not right?

[Gabriel Genellina]

At Thursday 24/8/2006 17:44, 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).
>
>That is merely a logical use of OO after all when would a car and an
>orange be the same?

Uh... what's the point...?
By example, an orange inside a car would be modeled using 
composition, never inheritance.

>I was wondering more about the mechanics of Python: when does B1 show
>different characteristics than B2  (forgoing the obvious simple things,
>like 't' above).

Inheritance implies that *all* methods/attributes of A are exposed by 
B1; it's directly supported by the language. Inheritance is usually a 
relationship between classes. If you add a method foo() to A, 
instances of B1 automatically have it. A B1 instance "is an" A instance.
Using delegation, you have to delegate the desired method calls 
yourself (but there are ways to do that automatically, too). 
Delegation is a relationship between instances. If you add a method 
foo() to A, you have to add it to B2 too. A B2 instance "is not an" A instance.



Gabriel Genellina
Softlab SRL 


	
 p4.vert.ukl.yahoo.com uncompressed Thu Aug 24 21:27:05 GMT 2006 
	
		
__________________________________________________ 
Preguntá. Respondé. Descubrí. 
Todo lo que querías saber, y lo que ni imaginabas, 
está en Yahoo! Respuestas (Beta). 
¡Probalo ya! 
http://www.yahoo.com.ar/respuestas