When is a subclass not right?

[Gabriel Genellina]

At Thursday 24/8/2006 19:51, 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).
>>>
>>>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.
>
>Once again you answered all the generic things about classes. I 
>could have taken that from a book on OO. All well and good but not 
>specifically addressed to the question I asked. Please read what I 
>wrote. I am more interested in knowing specifics about the Python 
>implementation and if there are any "gotchas" that would make B1 
>different from B2.

Please stay on the list.

b1 = B1()
b2 = B2()
isinstance(b1, A) -> True
isinstance(b2, A) -> False

For any other method defined in A, say foo:
b1.foo() is OK
b2.foo() raises AttributeError

So you decide to override foo() too (in both implementations, B1 and B2)
Let's say, A.bar() calls self.foo()
b1.bar() calls B1.foo on b1
b2.bar() calls A.foo on t - B2.foo is *not* called.

Enough examples? Inheritance and delegation are *not* the same thing. 
Anyway, none of these examples is very Python-specific.



Gabriel Genellina
Softlab SRL 


	
 p5.vert.ukl.yahoo.com uncompressed Fri Aug 25 01:27:04 GMT 2006 
	
		
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