Lesson 25
The object-oriented thought process
Python Mini-Course
University of Oklahoma
Department of Psychology
1 Python Mini-Course: Lesson 25 6/16/09

Lesson objectives
1.
2.
3.
Define the key terms used in objectoriented programming (OOP)
Understand the difference between an object and a class
Describe the types of relationships that are possible between objects
2 Python Mini-Course: Lesson 25 6/16/09

Procedural vs. OOP

Review from Lesson 6

Procedural programming separates the program operations and the data

Object-oriented programming packages the program operations and the data together in object
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What is an object?

The building blocks of an O-O program

A program that uses O-O is basically a collection of objects

Objects interact much like things in the real world do
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What is an object?

Objects have two components:

Data (i.e., attributes)

Behaviors (i.e., methods)
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Object attributes

Store the data for that object

Example (taxi):

Driver

OnDuty

NumPassengers

Location
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Object methods

Define the behaviors for the object

Example (taxi):

PickUp

DropOff

GoOnDuty

GoOffDuty
• GetDriver
• SetDriver
• GetNumPassengers
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Object interface

To use a method, the user
(programmer) must know:

Name of the method

Parameters to pass to the method

What (if anything) the method returns
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Object implementation

The user does NOT need to know how the method works internally
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What is a Class?

A blueprint for an object

Classes can be thought of as templates or cookie cutters

Given a class description, we can
instantiate objects of that class

Classes are high-level data types
10 Python Mini-Course: Lesson 25 6/16/09

OOP concepts

Encapsulation

Data and behaviors are packaged together, but the object only reveals the interfaces needed to interact with it

Internal data and behaviors can remain hidden
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OOP concepts

Interfaces

Fundamental means of communication between objects

Should completely describe to user
(programmer) how to interact with the object

Should control access to attributes
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Inheritance

You can create new classes by abstracting out common attributes and behaviors from a parent (or base) class
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Is-a relationship

Because sub-classes inherit from their base class, they have an isa relationship:

Lion is a cat

Cat is a mammal
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Polymorphism

Allows similar objects to to respond to the same message
(method call) in different manners

Sub-classes can override base class methods
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OOP example: animals.py
class Animal: def __init__(self, name): # Constructor of the class self.name = name class Cat(Animal): def talk(self): return 'Meow!' class Dog(Animal): def talk(self): return 'Woof! Woof!'
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Composition

Objects can contain other objects

This is called a has-a relationship

Example:

Taxi has-a driver
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