You've got it right Brian. It is this ability to change the way *existing* code works that makes OOP more than a neat way to package data and functions. This is polymorphism at its most powerful.
Its why in a C++/Java world the convention is to write hook methods that are called by the public methods. Thus sub classes can override the hook methods and change the behaviour of the public methods, without changing the basic sequence of processing. The common example here is a shape heirarchy: Shape defines a move method. move() calls delete() and draw() hooks. New classes override delete and draw and get move for free because it will always call the hooks in the right sequence and each subclass will actually do the appropriate work. Its another reason why you should never refer to an object or method *calling* another method (as Pilgrim does). Rather think of the method sending a *message* to the self object which invokes the appropriate method. This decoupling of message from method is a vitally important concept in OO and onvce you understand the concept of messages being sent resulting in methods being invoked OOD becomes much easier to grasp. This is explicit in Lisp, Smalltalk and Objective C where methods may have completely different names to messages, and multiple messages may invoke the same method - possibly with different default parameters - a very powerful technique! it's just about possible to do this in Python with lambdas etc but its messy. Alan G. ----- Original Message ----- From: "Brian van den Broek" <[EMAIL PROTECTED]> To: "Tutor" <[EMAIL PROTECTED]> Sent: Saturday, December 18, 2004 1:31 PM Subject: [Tutor] class overriding question > Hi all, > > instead of sleeping, I've been up all night finally attacking my > apprehension about classes. I think I'm mostly getting the hang of it -- > I managed to convert a 300 line procedural script into (what I think is) > a fully object-oriented approach. :-) > > I made a lot of use of Mark Pilgrim's Dive Into Python > <http://diveintopython.org/>, but Pilgrim said something that I'd like > to check my understanding of. In section 5.5 > <http://diveintopython.org/object_oriented_framework/userdict.html> he > writes: > > > Guido, the original author of Python, explains method overriding this > > way: "Derived classes may override methods of their base classes. > > Because methods have no special privileges when calling other methods > > of the same object, a method of a base class that calls another > > method defined in the same base class, may in fact end up calling a > > method of a derived class that overrides it. (For C++ programmers: > > all methods in Python are effectively virtual.)" If that doesn't make > > sense to you (it confuses the hell out of me), feel free to ignore > > it. I just thought I'd pass it along. > > I think I get this, but my inexperience with classes and Pilgrim's > rhetorical flourish make me doubt myself. I think the following three > ways of saying it all say the same thing (to a greater or lesser degree > of precision) as the quote from Guido above. Do I have the idea? > > Say class spam defines a method ham, and a method eggs, where ham calls > eggs. Further say class foo is derived from spam and overrides its eggs > method, but not its ham method. Then calling the ham method of an > instance bar of foo (and thus calling the ham method of spam, as foo is > a spam), will call the eggs method of foo, despite the fact that ham is > a method of spam, and within spam points originally to spam's version of > the eggs method. > > Alternatively, when calling bar.ham(), Python essentially says "OK, > bar's a foo. Does foo have a ham method? No, but it is derived from > spam. Does spam have a ham method? Yes, and it calls an eggs method. > Since bar's a foo, I should first look to see if foo has an eggs method. > (Nevermind that it was code in spam that started me off looking for > eggs.) Golly good, it does. So I will use that and not even bother to > look at spam's version of eggs." > > One other way: if we represent class inheritance as a tree, an a given > instance is of a class at level n in the tree, any search for a method > begins with the instance class and works up the tree to the ultimate > base class at level 1, no matter how high up the tree the search for the > method was initiated. And, in that search up the tree, the first > correctly named method to be found will be used. (I'm using the > mathematician's notion of trees with roots at the top.) > > Anyway, I hope I've both made sense and have got the idea right. > > Best to all, > > Brian vdB > > _______________________________________________ Tutor maillist - [EMAIL PROTECTED] http://mail.python.org/mailman/listinfo/tutor
