Introduction to Python

1
Introduction to Python

• Object Oriented Programming

2
Basics

• Motivation
• Reuse code
• Separate implementation details from interface
• Idea
• Model code after real-world objects
• Group software into classes
• Encapsulate information into an object

3
Abstraction

• Hide implementation details
• Only expose the interface
• Change the implementation without affecting
  interface

Interface
Implementation
4
Characteristics

• Objects have 2 major characteristics
• State
• A set of data encapsulated by the object
• The variables containing the objects current
  state
• Behavior
• A set of methods which affect the state of the
  object
• The procedures/functions which operate on the data

5
State

• The car has a state (set of variables or members)
• The amount of gas in the tank
• The pressure of the break on the tire
• The amount of gas going into the engine
• The angle of the tires

6
Behaviour

• There are ways to change the behaviour of the car
• Add gas to the tank
• Step on the break pedal
• Step on the gas pedal
• Turn the steering wheel
• These methods make up the interface

7
Classes

• A class is the blueprint for an object
• Defines how to create the object
• Defines the interface
• Defines the state variables to be used

Class
Object
8
Objects

• An object is an instance of a class
• There can be many instances of a class
• Each instance has its own state
• Changing one instances does not affect others

9
Car Interface

• The cars interface can be expressed in code

Methods
Members
Accelerate(amount) Decelerate(amount) AddGas(amoun
t) Steer(direction, amount) GetSpeed() GetRemainin
gGas()
gas currentWheelDirection amountAcceleration amoun
tDeceleration previousSpeed
10
Creating Objects

• All objects must be explicitly created
• Initialization takes place in the constructor
• Variables can be passed to the constructor to
  control how the object is created
• When the object is no longer needed it is deleted
  by the destructor
• Python does not require user-defined destructors
• Python Example
• car1 Car("Firebird", "red")
• car2 Car("Blazer", "silver")
• car1.Accelerate(2)
• print car1.GetSpeed()
• 10 m.p.h.
• print car2.GetSpeed()
• 0 m.p.h.

11
Using Objects

• Methods are called on instances
• The class name cannot be used to call methods
• Example
• car1 Car("Firebird", "red")
• Car.Accelerate(2) Error! Car is the name
  of the class
• car1.Accelerate(2) Correct! car1 is an
  instance of Car

This is not wrong if the method is a static
method
12
Advantages

• Code Reuse
• By packaging code into a class, code can be more
  easily shared
• Abstraction
• Details of the implementation can be changed
  without affecting client code
• Simplicity
• Simplify complex tasks by creating a simpler
  interface

13
Turtle Class

• Example Class Turtle
• A Logo-like drawing tool
• The turtle can rotate in any direction and draw
  straight lines

14
Turtle Class

• Basic Interface
• DrawForward(num)
• MoveForward(num)
• Rotate(amount, direction, units)
• Basic Members
• currentPosition
• currentAngle
• canvas

15
Turtle Class
currentPosition (50, 6) currentAngle 0.785
16
Turtle Class

• Advanced Interface
• SaveState()
• RecoverState()
• SetColor(color)
• SetThickness(thickness)
• Advanced Members
• stateStack
• currentColor
• currentThickness

17
Turtle Example
import TurtleDraw myTurtle TurtleDraw.Turtle()
Construct a Turtle object myTurtle.DrawForward(1
00) myTurtle.SaveState() Save current
position and angle myTurtle.Rotate(45) myTurtle.Dr
awForward(75) myTurtle.RecoverState() Return
to saved position and angle myTurtle.Rotate(-45) m
yTurtle.DrawForward(75) TurtleDraw.run()
Required by Tkinter to draw
18
Turtle Example
19
Multiple Turtles

• Multiple turtles can be created
• Each has its own members and state
• Each is controlled separately

20
Multiple Turtle Example
import TurtleDraw Create two different
Turtles redTurtle TurtleDraw.Turtle(color'red')
blueTurtle TurtleDraw.Turtle(color'blue') red
Turtle.Rotate(90) blueTurtle.Rotate(-90) redTurtl
e.DrawForward(100) blueTurtle.DrawForward(50) Tur
tleDraw.run() Required by Tkinter to draw