Python Programming - VII. Customizing Classes and Operator Overloading

SEQUENCE OVERLOADING
However, too much overloading may also backfire. 

VII. CUSTOMIZING CLASSES

PYTHON PROGRAMMING

Engr. Ranel O. Padon

PYTHON PROGRAMMING TOPICS
I

•Introduction to Python Programming

II

•Python Basics

III

•Controlling the Program Flow

IV

•Program Components: Functions, Classes, Packages, and Modules

V

•Sequences (List and Tuples), and Dictionaries

VI

•Object-Based Programming: Classes and Objects

VII

•Customizing Classes and Operator Overloading

VIII

•Object-Oriented Programming: Inheritance and Polymorphism

IX

•Randomization Algorithms

X

•Exception Handling and Assertions

XI

•String Manipulation and Regular Expressions

XII

•File Handling and Processing

XIII

•GUI Programming Using Tkinter
2

INTRODUCTION
Customization provides solutions for various needs.

INTRODUCTION

Customization has pros and cons.

INTRODUCTION

Special Methods: __init__ and __del__
__init__ creates objects
__del__ destroys objects

5

INTRODUCTION
The typical method-call notation is cumbersome for
mathematical classes.
>> polynomial1.add(polynomial2)
better or more natural way:

>> polynomial1 + polynomial2
This is called Operator Overloading.

6

INTRODUCTION

For faster development, reuse, modify, or extend
the built-in attributes, methods & operators of a Class.

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INTRODUCTION

Python enables the programmer to overload most operators
to be sensitive to the context in which they are used.
>> print 2 + 3
>> print “Mang” + “Jose”
>> print 2 * 3
>> print “Mang” * 2

#prints “MangMang”

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INTRODUCTION

The interpreter takes the appropriate action based
on the manner in which the operator is used.

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INTRODUCTION

Some operators are overloaded frequently,
especially operators like + and —.
The job performed by overloaded operators
also can be performed by explicit method calls,
but operator notation is often clearer.

10

STRING REPRESENTATION

A Python class can define special method __str__,
to provide an informal (i.e., human-readable) string
representation of an object of the class.
__str__ is analogous to Java’s toString() method.

11


If a client program of the class contains the statement
print objectOfClass
Python calls the object’s __str__ method and outputs the
string returned by that method.

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The raw face of an object.

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default invocation
>> print halimawSaBanga
__main__.HalimawSaBanga instance at 0x0204E120

if __str__ is overriden
>> print halimawSaBanga
500-year-old na Halimaw natagpuan sa banga!

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The Raw Faces of an Object

The __str__ Faces of an Object

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OPERATOR OVERLOADING

Python does not allow new operators to be created, it does
allow most existing operators to be overloaded
when these operators are used with objects of a
programmer-defined type, the operators have meaning
appropriate to the new types.

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Overloading: It’s about maximizing capabilities.

OPERATOR OVERLOADING

overloading contributes to the extensibility
of Python language

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OPERATOR OVERLOADING Restrictions

1. precedence cannot be changed
2. arity cannot be changed (i.e., unary to binary)

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OPERATOR OVERLOADING Restrictions

28

OPERATOR OVERLOADING Unary

A unary operator for a class is overloaded as a method
that takes only the object reference argument (self )

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OPERATOR OVERLOADING Unary

30

OPERATOR OVERLOADING Binary

A binary operator or statement for a class is overloaded as a
method with two arguments: self and other.

31

OPERATOR OVERLOADING Rational

A Rational number is a fraction represented as a numerator
(top) and a denominator (bottom).

A rational number can be positive, negative or zero.
Class Rational’s interface includes a default constructor, string
representation method, overloaded abs function, equality
operators and several mathematical operators.

32

OPERATOR OVERLOADING Rational

33


When overloading binary operator +, if y and z are objects of
class Rational, then y + z is treated as if y.__add__(z) had
been written, invoking the __add__ method.

34


Usually, overloaded binary operator methods create and return
new objects of their corresponding class.

36


What happens if we evaluate the expression y + z or the
statement y += z, and only y is an object of class Rational?
In both cases, z must be coerced (i.e., converted) to an
object of class Rational, before the appropriate operator
overloading method executes.

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OPERATOR OVERLOADING Built-ins

A class also may define special methods that execute when
certain built-in functions are called on an object of the class.
For example, we may define special method __abs__
for class Rational, to execute when a program calls
abs(rationalObject) to compute the absolute value
of an object of that class.

40

OPERATOR OVERLOADING Built-ins

41

TYPE CONVERSION

Sometimes all the operations “stay within a type.”

For example, adding (concatenating) a string to a string
produces a string. But, it is often necessary to convert or
coerce data of one type to data of another type.

42

TYPE CONVERSION

Programmers can force conversions among built-in types
by calling the appropriate Python function, such as int or
float.

43

TYPE CONVERSION

But what about user-defined classes?

The interpreter cannot know how to convert among userdefined classes and built-in types.

44

TYPE CONVERSION

The programmer must specify how such conversions are to
occur with special methods that override the appropriate
Python functions.

45

TYPE CONVERSION

For example, a class can define special method __int__
that overloads the behavior of the call int( anObject ) to
return an integer representation of the object.

46

CASE STUDY: RATIONAL CLASS
Sample computations with Rational objects:

49


function gcd() computes the greatest common divisor of two values.
Class Rational uses this function to simplify the rational number.
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END NOTES

Overloading is a powerful concept.
It allows the programmer to reuse/modify existing concepts
and operations.

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…and enables the customization of classes.


However, too much overloading may also backfire. 

REFERENCES
 Deitel, Deitel, Liperi, and Wiedermann - Python: How to Program (2001).

 Disclaimer: Most of the images/information used here have no proper source citation, and I do
not claim ownership of these either. I don’t want to reinvent the wheel, and I just want to reuse
and reintegrate materials that I think are useful or cool, then present them in another light,
form, or perspective. Moreover, the images/information here are mainly used for
illustration/educational purposes only, in the spirit of openness of data, spreading light, and
empowering people with knowledge. 

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Python Programming - VII. Customizing Classes and Operator Overloading

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