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Data Structures and Algorithms
with Object-Oriented Design Patterns in Python |
The _compareTo method given in Program 
is an abstract method.
In particular, the body of the method does nothing.
It is expected that the _compareTo method will be overridden
in a class derived from the Object class.
However, this is only a convention--the Python language does
nothing to ensure that the method is overridden.
And if the method is not overridden,
it remains possible for a Python program to call the abstract method.
To prevent the calling of abstract methods,
we introduce the abstractmethod
descriptor class
shown in Program .
The idea is to wrap a function object
that is supposed to be an abstract method
in an instance of the abstractmethod class
as shown on line 17 of Program .
Program: abstractmethod class definition.
The abstractmethod class does its magic by defining a special __get__ method. Python uses the __get__ method of a class to bind a method to an instance. For example, suppose you have an instance obj of some class Cls. When you call a method like this:
obj.func(*args)Python effectively does this:
meth = __get__(func, obj, Cls) meth.__call__(*args)
In Program ,
we implement a __get__ method
that returns an instance of
the nested abstractmethod.method class.
This class in turn implements a __call__ method
that raises a TypeError exception.
The effect of this is that whenever an abstract method is called
an exception is raised.
Copyright © 2003, 2004 by Bruno R. Preiss, P.Eng. All rights reserved.