Object Slides - jude-lindale/Wiki GitHub Wiki

Objects

• An object is a structure for representing a concept, thing, or situation
• Attached to an object are definitional and descriptive information and how the object behaves

Classes

• Blueprint or description of each object
• the blueprint holds information that describes the object
• Definitional
  • E.G. A square always has 4 sides
• Descriptive but allowed to change.
  • E.G. A Swedish person has blue eyes.
• Behavior.
  • E.G. A child knows how to walk.

Inheritance

• Key aspect
• Allows you to build a hierarchy of objects
  • They inherit descriptions and functions from each other.
    • E.G. A dining table inherits properties from table but has extra properties of its own.

Instances

• Once you have defined the Class
• You can make instances
  • My dining table is an instance of the dining table blueprint or class.
  • Instances start with all the defined values and behaviors of the blueprint

Defining a Class

• When creating a class, consider;

  • all of its attributes
  • the tasks you will need to do with the class
  • the communication that will initiate those tasks

• You construct a class in two parts

  • a declaration section
  • implementation section, which contains the functions themselves

• when creating an object you are creating an instantiation of a class

  class Pet:
    legs = 4
   def talk(self):
     print("I am a virtual pet")
  

Class Attributes

• Defined at the top level

• Variable that is the same value for all instances

• Can be changed bu it changes for all

  legs = 4
  Totalnumberofinstancescreated = 0
  kind = 'canine'
  

Instance Methods

   def talk(self):
     print("I am a virtual pet")

• regurlar instance method
• takes one parameter, self, (at least)
• Using self methods access attributes and other methods
• They modify an object's state
• Can also access the class itself through the self.__class__ attribute

Class Methods

   @classmethod
   def hello(cls):
       print("Hello")

• @classmethod decorator flag
• Can modify class but not instance state

Static Methods

   @staticmetho
    def staticmethod():

    return 'static method called'

• called on a class
• do not have an associated object
• can neither modify object state nor class state

Instance Definition in Python

Having defined the pet

   class Pet:
        def talk(self):
            print("I am a virtual pet")
pet1 = Pet() #instantiation
pet1.talk() #method invocation

Constructors

• Special method that all classes have that is called on instance creation

• usually called with new

• in python

   _init_(self)
  

• there are several built in python methods

  • start and end with _
  • E.g. str to print something about an object

Example

class Critter(object):
        def __init__(self, name):
        print "A new critter has been born!"
        self.name = name

    def __str__(self):
        rep = "Critter object\n"
        rep += "name: " + self.name + "\n"
        return rep
    def talk(self):
    print "Hi.  I'm", self.name, "\n"

Data Hiding and Encapsulation

• A major asset of object-oriented programming is that information hiding is more complete than with the procedures used in procedural programming

  • You employ a primitive form of information hiding when you write a program using functions to which you know only the interface

• private

  • Inaccessible to functions not part of the class

• Once a class is created, no outside function over which you have no control can ever modify or use private member data of any object in the class in an erroneous manner

• When you create and test a class, and store its definition in a file, programs using that definition can be prevented from using member data incorrectly

• Usually, you want at least some functions to be public

  • Accessible by both member and nonmember functions
  • Clearly defined Interface Python and privacy

• Private attributes/methods start with __

• (double underscore)

   Self.name=name #public
   __Self._mood=mood #private
   Def __privatemethod(self)
  

get-set methods

def get_name(self):
        return self.__name

def set_name(self, new_name):
        if new_name == "":
            print "A critter's name can't be the empty string."
        else:
            self.__name = new_name
            print "Name change successful."