Introduction%20to%20Computer%20Programming%20Chapter%208:%20Software%20Objects

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Introduction to Computer Programming Chapter 8
Software Objects

• Michael Scherger
• Department of Computer Science
• Kent State University

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Contents

• Object oriented programming
• Creating classes, methods and objects
• Constructors
• Attributes
• Class attributes and static methods
• Object encapsulation
• Private attributes and methods
• Controlling attribute access

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Critter Caretaker

• Example Critter Caretaker Program

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Object Oriented Programming

• OOP is a different way of thinking about
  programming
• Basic building block is an software object
• Objects are created from classes
• A template or blueprint for making objects
• Two objects built from the same class can have
  different values for their attributes
• Classes define attributes and methods

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Creating Classes, Methods, and Objects

• Defining a Class
• A class must be defined before any objects can be
  created from it
• Classes start with the reserved word class
• Example
• class Critter(object)
• This is a Critter object

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Creating Classes, Methods, and Objects

• Defining a Method
• A method is a type of function used by objects
• Methods have similar syntax structure to
  functions
• Methods have a special parameter called self by
  convention
• It provides a way for a method to refer to the
  object itself
• Example
• def talk( self )
• print Hi, Im an instance of a Critter Class

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Creating Classes, Methods, and Objects

• Instantiating an Object
• Once your class (blueprint) is finalized, you
  must create an instance of the class (an object)
• This instructs Python to use the blueprint and
  creates an object
• Example
• crit Critter()

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Creating Classes, Methods, and Objects

• Invoking a Method
• Once the object has been instantiated, you can
  access the attributes and invoke the methods
• Attributes and methods can be accessed using dot
  notation
• Example
• crit.talk()

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Creating Classes, Methods, and Objects

• Example Simple Critter Program

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Constructors

• Creating a Constructor
• A constructor is a special method that is
  automatically invoked right after the new object
  is created
• A start up method usually used to initialize
  attribute values of an object
• Example
• def __init__(self, name)
• print A new critter has been born

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Constructors

• Creating Multiple Objects
• Once your class has been defined, your program
  can create multiple objects
• Example
• main program
• crit1 Critter()
• crit2 Critter()
• crit1.talk()
• crit2.talk()

• A critter has been born!
• A critter has been born!
• Im an instance of class Critter
• Im an instance of class Critter

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Constructors

• Example Constructor Critter Program

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Attributes

• Initializing Attributes
• Attributes are special properties of an object
  that are defined by a class
• Example
• def __init__(self, name)
• print A new critter has been born!
• self.name name

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Attributes

• Accessing Attributes
• Attributes are accessed in the same manner
  methods are accessedusing dot notation
• Example
• print crit1.name

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Attributes

• Printing an Object
• By default, printing an object in Python would
  print something very cryptic
• lt__main__.Critter object at 0x0123ABCFgt
• A class can define a special print method
  called __str__ that can be used to print a class
• Example
• def __str__(self)
• print Hi, Im, self.name, \n

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Attributes

• Example Attribute Critter Program

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Class Attributes and Static Methods

• Creating a Class Attribute
• Sometimes your class definition need to have
  attributes and methods that are shared by all
  objects of that class
• Example is a count of the number of objects of a
  class (reference count)
• Class attributes are any assignment statements in
  a class definition yet outside of any class method

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Class Attributes and Static Methods

• Example Classy Critter Program

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Class Attributes and Static Methods

• Creating a Class Attribute
• Example
• class AnyClass(object)
• total 0
• other class methods...
• A class attribute assignment statements are only
  executed once
• When Python first sees the definition
• Class attributes can be used without any objects
  created

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Class Attributes and Static Methods

• Accessing a Class Attribute
• Accessing a class attribute is the same as
  accessing a regular attribute
• Use dot notation
• Example
• print AnyClass.total

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Class Attributes and Static Methods

• Creating a Static Method
• A static method is a method common to all objects
• Does not contain the reserved word self in the
  parameter list
• Uses the function staticmethod() to declare that
  a function is a static method
• See example in Classy Critter Program

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Class Attributes and Static Methods

• Invoking a Static Method
• Static methods can be invoked in the same manner
  as attributes and other methods
• Uses dot notation
• Can be invoked prior to any object instantiation

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Object Encapsulation

• Just as functions encapsulate details about a
  procedure, objects also encapsulate details into
  a single container-like structure
• Example checking accounts

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Private Attributes and Methods

• Example Private Critter Program

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Private Attributes and Methods

• Creating Private Attributes
• Many times the designer of a class would like the
  some (or most) of the attributes and methods to
  be private or not directly accessible to the user
• Attributes can be either public or private
• public attributes are directly accessible by the
  client
• private attributes are not directly accessible by
  the client
• some other class method must access them

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Private Attributes and Methods

• Creating Private Attributes
• Private attributes are created by prepending two
  underscores in front of the attribute name
• Example
• def __init__(self, name, mood)
• print A new critter has been born!
• self.name name public attribute
• self.__mood mood private attribute

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Private Attributes and Methods

• Accessing Private Attributes
• Private attributes can only be accessed inside
  the class definition
• Example
• crit Critter( name Chelsea, mood
  grumpy)
• print crit.mood error AttributeError
• print crit.__mood error AttributeError
• print crit._Critter__mood OK

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Private Attributes and Methods

• Creating Private Methods
• Similar to private attributes, private methods
  have two underscores prepended in front of them
• Example
• def __some_private_method(self)
• print This is a private method

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Private Attributes and Methods

• Accessing Private Methods
• Private methods can only be invoked inside a
  class definition
• Example
• crit Critter( name Chelsea, mood
  grumpy)
• print crit.some_private_method NO
• print crit.__some_private_method NO
• print crit._Critter__some_private_method OK

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Private Attributes and Methods

• Respecting an Objects Privacy
• It is good programming practice to behave
  yourself and respect the private and public
  properties of an attribute or method

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Private Attributes and Methods

• Understanding When to Implement Privacy
• When you create a class
• Create methods so that clients wont need to
  directly access an objects attributes
• Use privacy sparingly and only for those few
  attributes and methods that are completely
  internal to the operation of the object
• When you use an object
• Minimize the direct reading of an objects
  attributes
• Avoid directly altering an objects attributes
• Never directly access an objects private
  attributes or methods

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Controlling Attribute Access

• Example Property Critter Program

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Controlling Attribute Access

• Using Get Methods
• Rather than reading an objects attributes
  directly, implement a get method that returns
  the value of the attribute
• Make the attribute privatebut the function to
  retrieve the value public
• Example
• def __init__(self, name)
• print A new critter has been born!
• self.__name name
• def get_name(self)
• return self.__name

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Controlling Attribute Access

• Using Set Methods
• Rather than writing to an objects attribute
  directly, implement a set method that writes
  the value to the attribute
• Make the attribute privatebut the function to
  update the value public
• Example
• def set_name(self, new_name)
• if new_name
• print A critters name cannot be empty
• else
• self.__name new.name
• print Name change successful

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Controlling Attribute Access

• Using Properties
• A property allow you to use the access methods
  while hiding the details from the user
• It wraps the access methods around the
  consistent dot notation
• Example
• define a class
• define access methods get and set
• name property( get_name, set_name)

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Controlling Attribute Access

• Using Properties
• Example
• print crit.name calls the get_name method
• crit.name George calls the set_name method

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Critter Caretaker (Again)

• Example Critter Caretaker Program