Python Mini-Course

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Lesson 25The object-oriented thought process

• Python Mini-Course
• University of Oklahoma
• Department of Psychology

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Lesson objectives

1. Define the key terms used in object-oriented
   programming (OOP)
2. Understand the difference between an object and a
   class
3. Describe the types of relationships that are
   possible between objects

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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

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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 is-a 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