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Barb EricsonGeorgia Institute of Technology

July 2009

Introduction to Object-Oriented Programming in Python

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Learning Goals

• Compare procedural programming and object-oriented programming

• Introduce computation as simulation• Create objects in Python• Invoke methods on objects in Python• Create a subclass• Create a method in a class• Pass a parameter to a method in a class

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Procedural Programming

• Focus is on the procedures– The tasks that need to be done– Break large tasks into smaller tasks

• Write a function for each task

• Problems with procedural– The data is passed around and any function

can change the data– It is hard to find the function

• They can be in many files with many names

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Object-Oriented Programming• The focus is on who does each task as

well as the tasks to be done– Responsibility driven

• Advantages– Manage complexity by distributing

responsibility across objects– Make systems robust by making objects work

independently– Support reuse because objects provide

services

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Computers as Simulators

• “The computer is the Proteus of machines. Its essence is its universality, its power to simulate. Because it can take on a thousand forms and serve a thousand functions, it can appeal to a thousand tastes.” Seymour Papert in Mindstorms

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Creating a Simulation

• Computers let us simulate things– We do this by creating models of the things

we want to simulate– We need to define what types of objects we

will want in our simulation and what they can do

• Classes define the types and create objects of that type

• Objects act in the simulation

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Creating objects

• We have been creating objects in Python using– makePicture(file)– makeSound(file)

• But, these are functions we created to make it easy to create objects from these classes

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Creating objects in python• Use ClassName(value1,value2,…)

– Picture(file) # create a picture– Sound(file) # create a sound– World() # create a world

• Name the objects that are created>>> earth = World()>>> tina = Turtle(earth)>>> sue = Turtle(earth)

• Use the name to ask the object to do something objName.function()>>> tina.forward()

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Turtle Behaviors• forward() # forward by 100 pixels• forward(amount) # forward by amount• turnLeft() # 90 degree left• turnRight() # 90 degree right• turn(degrees) # pos right and neg is left • penUp() # pick up the pen• penDown() # put pen down• moveTo(x,y) # move to x and y location• setColor(blue) # set the color• setPenWidth(width) # size of pen trail• setVisible(false) # don't draw turtle

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Challenge

• Use a turtle to draw a square• Use a turtle to draw blocks of color using a

wide pen trail

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Create a Subclassclass SmartTurtle(Turtle): def drawSquare(self): for i in range (0 ,4): self.turnRight () self.forward ()

>>> earth1 = World()>>> smarty = SmartTurtle(earth1)>>> smarty.drawSquare()

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Passing Parameters to Methodsclass SmartTurtle(Turtle):

def drawSquare(self): for i in range (0 ,4): self.turnRight () self.forward ()

def drawSquare(self, width): for i in range (0 ,4): self.turnRight () self.forward(width)

>>> mars = World ()>>> tina = SmartTurtle(mars)>>> tina.drawSquare (30)>>> tina.drawSquare (150)>>> tina.drawSquare (100)

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Creating the Slide Classclass Slide: def show(self): show(self.picture) blockingPlay(self.sound)

>>> slide1=Slide()>>> slide1.picture =

makePicture(getMediaPath("barbara.jpg"))>>> slide1.sound = makeSound(getMediaPath("bassoon-c4.wav"))>>> slide1.show()

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Constructors

class slide:def __init__(self , pictureFile ,soundFile ):self.picture = makePicture(pictureFile)self.sound = makeSound(soundFile)

def show(self ):show(self.picture)blockingPlay(self.sound)

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Testing the constructordef playSlideShow2 ():

pictF = getMediaPath("barbara.jpg")soundF = getMediaPath("bassoon -c4.wav")slide1 = slide(pictF ,soundF)pictF = getMediaPath("beach.jpg")soundF = getMediaPath("bassoon -e4.wav")slide2 = slide(pictF ,soundF)pictF = getMediaPath("church.jpg")soundF = getMediaPath("bassoon -g4.wav")slide3 = slide(pictF ,soundF)pictF = getMediaPath("jungle2.jpg")soundF = getMediaPath("bassoon -c4.wav")slide4 = slide(pictF ,soundF)slide1.show ()slide2.show ()slide3.show ()slide4.show ()

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Object-oriented with Pictures>>> pic=Picture(getMediaPath("barbara.jpg"))>>> pic.show()>>> pic.getWidth()>>> pic.getHeight()>>> pixels = pic.getPixels()>>> pixel = pic.getPixel(0,0)>>> color = pixel.getColor();>>> red = pixel.getRed()

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Picture Functionsdef show(picture ):

if not picture.__class__ == Picture:print "show(picture ): Input is not a picture"raise ValueError

picture.show ()

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Summary• Procedural programming has a focus on the

procedures (tasks)• Object-oriented programming has a focus on the

objects and on simulation• You can create objects from classes in Python

– Picture(file) or Class(parameters)• You can create subclasses in Python

– class SmartTurtle(Turtle):• You can overload methods by passing different

parameters