Lecture 8: Object Oriented Programming

Every Object has

• a type

• an internal data representation (primitive or composite)

• a set of procedures for interaction with the object

Object

an instance of a type; basically all data in Python

Object Oriented Programming (OOP)

• EVERYTHING IN PYTHON IS AN OBJECT (and has a type)

• can create new objects of some type

• can manipulate objects

• can destroy objects (explicitly using del or just “forget” about them)

“garbage collection”

python system will reclaim destroyed or inaccessible objects

What are objects?

a data abstraction that captures…

1. an internal representation through data attributes

2. an interface for interacting with object through methods (aka procedures/functions). Defines behaviors but hides implementation

Advantages of OOP

• bundle data into packages together with procedures that work on them through well-defined interfaces

• divide-and-conquer development: implement and test behavior of each class separately; increased modularity reduces complexity

• classes make it easy to reuse code: many Python modules define new classes; each class has a separate environment (no collision on function names); inheritance allows subclasses to redefine or extend a selected subset of a superclass’ behavior

Creating the Class involves

• defining the class name

• defining class attributes

• for example, someone wrote code to implement a list class

Using the class involves

• creating new instances of objects

• doing operations on the instances

• for example, L=[1,2] and len(L)

Defining your own types

• use the class keyword to define a new type

• similar to def, indent code to indicate which statements are part of the class definition

What are attributes?

data and procedures that “belong” to the class

Data attributes

• think of data as other objects that make up the class

• for example, a coordinate is made up of two numbers

methods (procedural attributes)

• think of methods as functions that only work with this class

• how to interact with the object

• for example you can define a distance between two coordinate objects but there is no meaning to a distance between two list objects

Defining how to create an instance of an class

• first have to define how to create an instance of object

• use a special method called __init_to initialize some data attributes

Actually Creating an Instance of a Class?

• data attributes of an instance are called instance variables

• don’t provide argument for self, Python does this automatically

What is a method?

procedural attribute, like a function that works only with this class

“.” operator

is used to access any attribute ( a data attribute of an object and a method of an object)

How to use a method

Print Representation of an Object

• uninformative print representation by default

• define a __str_ method for a class

• Python calls the str method when used with print on your class object

• you choose what it does!

isinstance()

use to check if an object is a instance of a particular class

Special Operators

allow you to customize classes and can add cool functionality to them; like print, can override these to work with your class; define them with double underscores before/after

The Power of OOP

• bundle together objects that share: common attributes and procedures that operate on those attributes

• use abstraction to make a distinction between how to implement an object vs how to use the object

• build layers of object abstractions that inherit behaviors from other classes of objects and create our own classes of objects on top of Python’s basic classes