Deitel - Python, How To Program.pdf

pythonhtp1_08.fm Page 261 Monday, December 10, 2001 6:49 PM
Chapter 8 Customizing Classes 261
Outline
8.1 Introduction
8.2 Customizing String Representation: Method __str__
8.3 Customizing Attribute Access
8.4 Operator Overloading
8.5 Restrictions on Operator Overloading
8.6 Overloading Unary Operators
8.7 Overloading Binary Operators
8.8 Overloading Built-in Functions
8.9 Converting Between Types
8.10 Case Study: A Rational Class
8.11 Overloading Sequence Operations
8.12 Case Study: A SingleList Class
8.13 Overloading Mapping Operations
8.14 Case Study: A SimpleDictionary Class
Summary Terminology Self-Review Exercises Answers to Self-Review Exercises Exercises
8.1 Introduction
In Chapter 7, we introduced the basics of Python classes and the notion of abstract data
types (ADTs). We discussed how methods __init__ and __del__ execute when an object
is created and destroyed, respectively. These methods are two examples of the many
special methods that a class may define. A special method is a method that has a special
meaning in Python; the Python interpreter calls one of an object’s special methods when
the client performs a certain operation on the object. For example, when a client creates an
object of a class, Python invokes the __init__ special method of that class.
A class author implements special methods to customize the behavior of the class. The
purpose of customization is to provide the clients of a class with a simple notation for
manipulating objects of the class. For example, in Chapter 7, manipulations on objects were
accomplished by sending messages (in the form of method calls) to the objects. This
method-call notation is cumbersome for certain kinds of classes, especially mathematical
classes. For such classes, it would be nice to use Python’s rich set of built-in operators and
statements to manipulate objects. In this chapter, we show how to define special methods
that enable Python’s operators to work with objects—a process called operator overloading.
It is straightforward and natural to extend Python with these new capabilities.
Operator overloading also requires great care, because, when overloading is misused, it can
make a program difficult to understand.
Operator + has multiple purposes in Python, for example, integer addition and string concatenation.
This is an example of operator overloading. The Python language itself overloads
operators + and *, among others. These operators perform differently to suit the context in
integer arithmetic, floating-point arithmetic, string manipulation and other operations.