Python - Object Oriented

Python has been an object-oriented language since it existed. Because of this, creating and using classes and objects are downright easy. This chapter helps you become an expert in using Python s object-oriented programming support.

If you do not have any previous experience with object-oriented (OO) programming, you may want to consult an introductory course on it or at least a tutorial of some sort so that you have a grasp of the basic concepts.

However, here is small introduction of Object-Oriented Programming (OOP) to bring you at speed −

Overview of OOP Terminology

Class − A user-defined prototype for an object that defines a set of attributes that characterize any object of the class. The attributes are data members (class variables and instance variables) and methods, accessed via dot notation.

Class variable − A variable that is shared by all instances of a class. Class variables are defined within a class but outside any of the class s methods. Class variables are not used as frequently as instance variables are.

Data member − A class variable or instance variable that holds data associated with a class and its objects.

Function overloading − The assignment of more than one behavior to a particular function. The operation performed varies by the types of objects or arguments involved.

Instance variable − A variable that is defined inside a method and belongs only to the current instance of a class.

Inheritance − The transfer of the characteristics of a class to other classes that are derived from it.

Instance − An inspanidual object of a certain class. An object obj that belongs to a class Circle, for example, is an instance of the class Circle.

Instantiation − The creation of an instance of a class.

Method − A special kind of function that is defined in a class definition.

Object − A unique instance of a data structure that s defined by its class. An object comprises both data members (class variables and instance variables) and methods.

Operator overloading − The assignment of more than one function to a particular operator.

Creating Classes

The class statement creates a new class definition. The name of the class immediately follows the keyword class followed by a colon as follows −

class ClassName:
    Optional class documentation string 
   class_suite

The class has a documentation string, which can be accessed via ClassName.__doc__.

The class_suite consists of all the component statements defining class members, data attributes and functions.

Example

Following is the example of a simple Python class −

class Employee:
    Common base class for all employees 
   empCount = 0

   def __init__(self, name, salary):
      self.name = name
      self.salary = salary
      Employee.empCount += 1
   
   def displayCount(self):
     print "Total Employee %d" % Employee.empCount

   def displayEmployee(self):
      print "Name : ", self.name,  ", Salary: ", self.salary

The variable empCount is a class variable whose value is shared among all instances of a this class. This can be accessed as Employee.empCount from inside the class or outside the class.

The first method __init__() is a special method, which is called class constructor or initiapzation method that Python calls when you create a new instance of this class.

You declare other class methods pke normal functions with the exception that the first argument to each method is self. Python adds the self argument to the pst for you; you do not need to include it when you call the methods.

Creating Instance Objects

To create instances of a class, you call the class using class name and pass in whatever arguments its __init__ method accepts.

"This would create first object of Employee class"
emp1 = Employee("Zara", 2000)
"This would create second object of Employee class"
emp2 = Employee("Manni", 5000)

Accessing Attributes

You access the object s attributes using the dot operator with object. Class variable would be accessed using class name as follows −

emp1.displayEmployee()
emp2.displayEmployee()
print "Total Employee %d" % Employee.empCount

Now, putting all the concepts together −

#!/usr/bin/python

class Employee:
    Common base class for all employees 
   empCount = 0

   def __init__(self, name, salary):
      self.name = name
      self.salary = salary
      Employee.empCount += 1
   
   def displayCount(self):
     print "Total Employee %d" % Employee.empCount

   def displayEmployee(self):
      print "Name : ", self.name,  ", Salary: ", self.salary

"This would create first object of Employee class"
emp1 = Employee("Zara", 2000)
"This would create second object of Employee class"
emp2 = Employee("Manni", 5000)
emp1.displayEmployee()
emp2.displayEmployee()
print "Total Employee %d" % Employee.empCount

Name :  Zara ,Salary:  2000
Name :  Manni ,Salary:  5000
Total Employee 2

emp1.age = 7  # Add an  age  attribute.
emp1.age = 8  # Modify  age  attribute.
del emp1.age  # Delete  age  attribute.

hasattr(emp1,  age )    # Returns true if  age  attribute exists
getattr(emp1,  age )    # Returns value of  age  attribute
setattr(emp1,  age , 8) # Set attribute  age  at 8
delattr(empl,  age )    # Delete attribute  age 

#!/usr/bin/python

class Employee:
    Common base class for all employees 
   empCount = 0

   def __init__(self, name, salary):
      self.name = name
      self.salary = salary
      Employee.empCount += 1
   
   def displayCount(self):
     print "Total Employee %d" % Employee.empCount

   def displayEmployee(self):
      print "Name : ", self.name,  ", Salary: ", self.salary

print "Employee.__doc__:", Employee.__doc__
print "Employee.__name__:", Employee.__name__
print "Employee.__module__:", Employee.__module__
print "Employee.__bases__:", Employee.__bases__
print "Employee.__dict__:", Employee.__dict__

Employee.__doc__: Common base class for all employees
Employee.__name__: Employee
Employee.__module__: __main__
Employee.__bases__: ()
Employee.__dict__: { __module__ :  __main__ ,  displayCount :
<function displayCount at 0xb7c84994>,  empCount : 2, 
 displayEmployee : <function displayEmployee at 0xb7c8441c>, 
 __doc__ :  Common base class for all employees , 
 __init__ : <function __init__ at 0xb7c846bc>}

a = 40      # Create object <40>
b = a       # Increase ref. count  of <40> 
c = [b]     # Increase ref. count  of <40> 

del a       # Decrease ref. count  of <40>
b = 100     # Decrease ref. count  of <40> 
c[0] = -1   # Decrease ref. count  of <40> 

#!/usr/bin/python

class Point:
   def __init__( self, x=0, y=0):
      self.x = x
      self.y = y
   def __del__(self):
      class_name = self.__class__.__name__
      print class_name, "destroyed"

pt1 = Point()
pt2 = pt1
pt3 = pt1
print id(pt1), id(pt2), id(pt3) # prints the ids of the obejcts
del pt1
del pt2
del pt3

3083401324 3083401324 3083401324
Point destroyed

class SubClassName (ParentClass1[, ParentClass2, ...]):
    Optional class documentation string 
   class_suite

#!/usr/bin/python

class Parent:        # define parent class
   parentAttr = 100
   def __init__(self):
      print "Calpng parent constructor"

   def parentMethod(self):
      print  Calpng parent method 

   def setAttr(self, attr):
      Parent.parentAttr = attr

   def getAttr(self):
      print "Parent attribute :", Parent.parentAttr

class Child(Parent): # define child class
   def __init__(self):
      print "Calpng child constructor"

   def childMethod(self):
      print  Calpng child method 

c = Child()          # instance of child
c.childMethod()      # child calls its method
c.parentMethod()     # calls parent s method
c.setAttr(200)       # again call parent s method
c.getAttr()          # again call parent s method

Calpng child constructor
Calpng child method
Calpng parent method
Parent attribute : 200

class A:        # define your class A
.....

class B:         # define your class B
.....

class C(A, B):   # subclass of A and B
.....

#!/usr/bin/python

class Parent:        # define parent class
   def myMethod(self):
      print  Calpng parent method 

class Child(Parent): # define child class
   def myMethod(self):
      print  Calpng child method 

c = Child()          # instance of child
c.myMethod()         # child calls overridden method

Calpng child method

#!/usr/bin/python

class Vector:
   def __init__(self, a, b):
      self.a = a
      self.b = b

   def __str__(self):
      return  Vector (%d, %d)  % (self.a, self.b)
   
   def __add__(self,other):
      return Vector(self.a + other.a, self.b + other.b)

v1 = Vector(2,10)
v2 = Vector(5,-2)
print v1 + v2

Vector(7,8)

#!/usr/bin/python

class JustCounter:
   __secretCount = 0
  
   def count(self):
      self.__secretCount += 1
      print self.__secretCount

counter = JustCounter()
counter.count()
counter.count()
print counter.__secretCount

1
2
Traceback (most recent call last):
   File "test.py", pne 12, in <module>
      print counter.__secretCount
AttributeError: JustCounter instance has no attribute  __secretCount 

.........................
print counter._JustCounter__secretCount

1
2
2