Python Programming: An Introduction to Computer Science

1
Python ProgrammingAn Introduction toComputer
Science

• Chapter 4
• Objects and Graphics

2
Objectives

• To understand the concept of objects and how they
  can be used to simplify programs
• To be familiar with some objects available in
  Zelle's graphics library
• To be able to create objects in programs and call
  appropriate methods to perform graphical
  computations

3
Overview

• Each data type can represent a certain set of
  values, and each had a set of associated
  operations
• The traditional programming view is that data is
  passive its manipulated and combined with
  active operations

4
Overview

• Modern computer programs are built using an
  object-oriented approach
• Most applications youre familiar with have
  Graphical User Interfaces (GUI) that provide
  windows, icons, buttons and menus
• Theres a graphics library (graphics.py) written
  specifically to go with this book. Its based on
  Tkinter

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The Object of Objects

• Basic idea view a complex system as the
  interaction of simpler objects. An object is a
  sort of active data type that combines data and
  operations
• Objects know stuff (contain data) and they can do
  stuff (have operations)
• Objects interact by sending each other messages

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The Object of Objects

• Suppose we want to develop a data processing
  system for a college or university.
• We must keep records on students who attend the
  school. Each student will be represented as an
  object.

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The Object of Objects

• The student object would contain data like
• Name
• ID number
• Courses taken
• Campus Address
• Home Address
• GPA
• Etc.

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The Object of Objects

• The student object should also respond to
  requests.
• We may want to send out a campus-wide mailing, so
  wed need a campus address for each student.
• We could send the printCampusAddress message to
  each student object. When the student object
  receives the message, it prints its own address.

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Object of Objects

• Objects may refer to other objects.
• Each course might be represented by an object
• Instructor
• Student roster
• Prerequisite courses
• When and where the class meets

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Sample Operations

• ista130 Course("CESL 102")
• ista130.addStudent('Jessie')
• ista130.addStudent('Casey')
• ista130.removeStudent('Jessie')
• ista130.changeRoom("CESL103")
• print(ista130.numberOfStudents())
• Guess the Output

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str objects

• str objects
• are immutable, cannot change them
• can be referenced with a variable
• aStrObject str('a string object')
• have data (not seen)
• a sequence of characters
• an integer as current number of characters
• locale like U.S., Germany, or China, ...
• have operations (methods)
• upped aStrObject.upper()

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Other str operations

• http//docs.python.org/library/stdtypes.htmlstrin
  g-methods
• s str('Jessie Casey') a str object
• s.count('e') Value?
• s.index('ssi') Value?
• s.index('Not') Value?
• s.endswith('sey') Value?
• s.split(' ') Value?

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Simple Graphics Programming

• This chapter uses the graphics.py library
  supplied with the supplemental materials
• Two location choices
• In Pythons Lib directory with other libraries
• In the same folder as your graphics program
  (what we'll do)
• like copying Rick's test modules

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Simple Graphics Programming

• Import all functions with '' and avoid using
  graphics. in graphics.graphWin()from graphics
  import
• A graphics window is a place on the screen where
  the graphics will appearwin GraphWin()
• What type of object is win?

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Simple Graphics Programming

• Windows can be closed/destroyed by issuing the
  commandwin.close()

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Simple Graphics Programming

• A graphics window is a collection of points
  called pixels (picture elements).
• The default GraphWin is 200 pixels tall by 200
  pixels wide (40,000 pixels total).
• One way to get pictures into the window is one
  pixel at a time, which would be tedious. The
  graphics routine has a number of predefined
  routines to draw geometric shapes.

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Simple Graphics Programming
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• The simplest object is the Point. Like points in
  geometry, point locations are represented with a
  coordinate system (x, y), where x is the
  horizontal location of the point and y is the
  vertical location.
• The origin (0,0) in a graphics window is the
  upper left corner.
• X values increase from right to left, y values
  from top to bottom.
• Lower right corner is (199, 199)

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Point and Circle objects

• from graphics import
• win GraphWin()
• p Point(50, 60)
• p.draw(win)
• p.getX() Value? ____
• p.getY() Value? ____
• circ Circle(p, 20)
• circ.draw(win)
• What is the size of the
• radius in pixels? _____

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Using Graphical Objects

• Computation is preformed by asking an object to
  carry out one of its operations
• In the previous examples we manipulated str,
  GraphWin, and Point
• These are examples of classes

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Using Graphical Objects

• The class describes the properties of the
  instance the data and operations
• We will write our own classes in chapter 10
• Each object is an instance of some class, If we
  say that Augie is a dog, then Augie is a specific
  individual in the larger class of all dogs.
• Augie is an instance of the dog class.

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Using Graphical Objects

• To create a new instance of a class, we use a
  special operation called a constructorltclass-name
  gt(ltparam1gt, ltparam2gt, )
• ltclass-namegt is the type of object
• s1 str('Leslie')
• p Point(50, 60)
• c Circle(p, 20)
• The arguments become the data remembered by the
  object
• To create a new instance of a class, we use a

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Using Graphical Objects

• Only the most relevant instance variables are
  shown (others include the color, window they
  belong to, etc.)

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Using Graphical Objects

• To perform an operation on an object, we send the
  object a message.
• The set of messages an object responds to are
  called the methods of the object.
• Methods are functions that live inside the
  object.
• Methods are invoked using dot-notationltobjectgt.lt
  method-namegt(ltparam1gt, ltparam2gt, )
• s.rjust(8,'') s is str object
• p.draw(win) p is a Point object
• fromLeft p.getX()
• c.setFill('yellow') c is a Circle object

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Circle remembers its Point

• circ Circle(Point(100, 100), 30)win
  GraphWin()circ.draw(win)

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Rectangle Objects

• upperLeft Point(50, 50)
• lowerRight Point(150, 80)
• rect Rectangle(upperLeft, lowerRight)
• rect.setFill('yellow')
• rect.setOutline('red')
• rect.draw(win)
• Is rect's height gt width ______
• What color is rect's border? ______
• How many pixels are yellow if the border
• is 1 pixel wide? ______

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The GraphWin object