Computer Science 111

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Computer Science 111

• Fundamentals of Programming I
• Introduction to Programmer-Defined Classes

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Objects, Classes, and Methods

• Every data value in Python is an object
• Every object is an instance of a class
• Built in classes include int, float, str, tuple,
  list, dict
• A class include operations (methods) for
  manipulating objects of that class (append, pop,
  sort, find, etc.)
• Operators (, , in, etc.) are syntactic
  sugar for methods

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What Do Objects and Classes Do for Us?

• An object bundles together data and operations on
  those data
• A computational object can model practically any
  object in the real (natural or artificial) world
• Some classes come with a programming language
• Any others must be defined by the programmer

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Programmer-Defined Classes

• The Turtle class is used to create objects that
  can draw pictures in a graphics window
• The Image class is used to load, process, and
  save images
• Like the built-in classes, these classes include
  operations to run with their instances

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Other Examples

• A Student class represents information about a
  student and her test scores
• A Rational class represents rational numbers and
  their operations
• A Die class represents dice used in games
• SavingsAccount, CheckingAccount, Bank, and ATM
  are used to model a banking system
• Proton, Neutron, Electron, and Positron model
  particles in nuclear physics

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The Die Class Its Interface and Use
Interface
die.py The module for the Die
class Die() Returns a new Die object roll()
Resets the die's value getValue()
Returns the die's value
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The Die Class Its Interface and Use
Interface
die.py The module for the Die
class Die() Returns a new Die object roll()
Resets the die's value getValue()
Returns the die's value
Use
from die import Die d Die()
Create a new Die object d.roll()
Roll it print(d.getValue()) Display its
value help(Die) Look up the
documentation
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Specifying an Interface

• The user of a class is only concerned with
  learning the information included in the headers
  of the classs methods
• This information includes the method name and
  parameters
• Collectively, this information comprises the
  classs interface
• Docstrings describe what the methods do

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Defining (Implementing) a Class

• The definition or implementation of a class
  includes completed descriptions of an objects
  data and the methods for accessing and modifying
  those data
• The data are contained in instance variables and
  the methods are called instance methods
• Related class definitions often occur in the same
  module

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Syntax Template for a Simple Class Definition
ltdocstring for the modulegt ltimports of other
modules usedgt class ltidentifiergt(ltparent
classgt) ltdocstring for the classgt
ltmethod definitionsgt
Basically a header followed by several method
definitions
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Defining the Die Class
from random import randint class
ltidentifiergt(ltparent classgt) ltdocstring for
the classgt ltmethod definitionsgt
Well use random.randint to roll the die
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The Class Header
from random import randint class Die(object)
ltdocstring for the classgt ltmethod
definitionsgt
By convention, class names are capitalized in
Python
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The Class Header
from random import randint class Die(object)
ltdocstring for the classgt ltmethod
definitionsgt
All Python classes are subclasses of the object
class A class can inherit behavior from its
parent class
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The Class Docstring
from random import randint class Die(object)
"""This class represents a six-sided die."""
ltmethod definitionsgt
A classs docstring describes the purpose of the
class
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Setting the Initial State
from random import randint class Die(object)
"""This class represents a six-sided die."""
def __init__(self) self._value 1
A method definition looks a bit like a function
definition The __init__ method (also called a
constructor) is automatically run when an object
is instantiated this method usually sets the
objects initial state (d Die())
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The self Parameter
from random import randint class Die(object)
"""This class represents a six-sided die."""
def __init__(self) self._value 1
The name self must appear as the first parameter
in each instance method definition Python uses
this parameter to refer to the object on which
the method is called
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Instance Variables
from random import randint class Die(object)
"""This class represents a six-sided die."""
def __init__(self) self._value 1
self must also be used with all instance method
calls and instance variable references within the
defining class self refers to the current object
(a die)
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Using Instance Variables
from random import randint class Die(object)
"""This class represents a six-sided die."""
def __init__(self) self._value 1
def roll(self) """Resets the die's
value.""" self._value randint(1, 6)
def getValue(self) return self._value
self._value refers to this objects instance
variable
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Where Are Classes Defined?

• Like everything else, in a module
• Define the Die class in a die module
• Related classes usually go in the same module
  (SavingsAccount and Bank the bank module)

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The Interface of the SavingsAccount Class
SavingsAccount(name, pin, bal) Returns a new
object getBalance() Returns the
current balance deposit(amount)
Makes a deposit withdraw(amount)
Makes a withdrawal computeInterest()
Computes the interest and
deposits it
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Defining the SavingsAccount Class
class SavingsAccount(object) """This class
represents a savings account.""" def
__init__(self, name, pin, balance 0.0)
self._name name self._pin pin
self._balance balance Other methods go
here
Note that name is a methods parameter, whereas
self._name is an objects instance variable
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The Lifetime of a Variable
class SavingsAccount(object) """This class
represents a savings account.""" def
__init__(self, name, pin, balance 0.0)
self._name name self._pin pin
self._balance balance Other methods go
here
Parameters exist only during the lifetime of a
method call, whereas instance variables exist for
the lifetime of an object
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Parameters or Instance Variables?

• Use a parameter to send information through a
  method to an object
• Use an instance variable to retain information in
  an object
• An objects state is defined by the current
  values of all of its instance variables
• References to instance variables must include the
  qualifier self

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The Scope of a Variable

• The scope of a variable is the area of program
  text within which its value is visible
• The scope of a parameter is the text of its
  enclosing function or method
• The scope of an instance variable is the text of
  the enclosing class definition (perhaps many
  methods)

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The Scope of a Variable
class SavingsAccount(object) """This class
represents a savings account.""" def
__init__(self, name, pin, balance 0.0)
self._name name self._pin pin
self._balance balance def deposit(self,
amount) self._balance amount def
withdraw(self, amount) self._balance -
amount
self._balance always refers to the same storage
area (for one object) amount refers to a
different storage area for each method call
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For Friday

• Continue in Chapter 8