[Tutor] class overriding question

[Kent Johnson]

Yup, that's right!

Attribute access (the dot operator '.') is an operation that happens at runtime, and each attribute 
access stands alone. Every attribute access goes through the same search path, starting with self, 
then the class (type) of self, finally the base classes. So, in your example, self.foo is found in 
the class of bar, while self.ham is found in the base class of the class of bar.

Kent

Brian van den Broek wrote:
> 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
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