Basic Inheritance Question

[Bruce Wolk]

Matthew Bell wrote:
> I've got a conceptual problem to do with inheritance.  
> I'd be grateful if someone could help to clear up my 
> confusion.
> 
> An example.  Say I need a class that's basically a 
> list, with all the normal list methods, but I want a 
> custom __init__ so that the list that is created is 
> [1,2,3] rather than [] (yes, it's a bogus example, 
> but it does to make the point).  Without bothering 
> with inheritance, I could do:
> 
>   class mysimplelistclass:
>     def __init__(self):
>       self.internallist = [1, 2, 3]
> 	
> This would work but I would, of course, need to define 
> methods in "mysimpleclass" to deal with all the various 
> methods that the original list class provides.
> 
> Obviously, the thing to do is to inherit from the list 
> class, override the __init__ method and leave the rest 
> of the normal list class's methods untouched.  So I'd 
> write something like:
> 
>   class myinheritedlistclass(list):
>     def __init__(self):
>       <now what?>
>     
> It's at this point I get confused.  Obviously, I don't 
> use the "self.internallist = [1, 2, 3]" bit as before 
> because I'd then need to override all of the rest of 
> the normal list methods to get them to act on 
> self.internallist.  
> 
> Conceptually, I suppose I need something like:
> 
> <somemagictoken> = superclass.self.__init__([1, 2, 3)]
> 
> but that is, of course, totally ridiculous.
> 
> Essentially, then, if I've inherited another class, how 
> do I create an instance of the class I've inherited such 
> that methods I haven't overrriden will still work, and 
> how can I then refer to that instance from my subclass?  
> I can guess it's something to do with "self" but exactly 
> what, I'm really at a loss.
> 
> Any assistance in my confusion would be gratefully received!
> 
> Regards,
>   Matthew.
> 
> 
I think this is what you want:

 >>> class MyList(list):
	def __init__(self,seq=[1,2,3]):
		super(MyList,self).__init__(seq)

		
 >>> a=MyList()
 >>> a
[1, 2, 3]
 >>> b=MyList([4,5])
 >>> b
[4, 5]
 >>> a + b
[1, 2, 3, 4, 5]
 >>>