[Tutor] class overriding question

[Brian van den Broek]

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