Python 3 - Object Oriented

Python has been an object-oriented language since the time it existed. Due to 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 a small introduction of Object-Oriented Programming (OOP) to help you −

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 is 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 an 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 the instances of a in 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/python3

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.salary = 7000  # Add an  salary  attribute.
emp1.name =  xyz   # Modify  age  attribute.
del emp1.salary  # Delete  age  attribute.

hasattr(emp1,  salary )    # Returns true if  salary  attribute exists
getattr(emp1,  salary )    # Returns value of  salary  attribute
setattr(emp1,  salary , 7000) # Set attribute  salary  at 7000
delattr(emp1,  salary )    # Delete attribute  salary 

#!/usr/bin/python3

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)

emp1 = Employee("Zara", 2000)
emp2 = Employee("Manni", 5000)
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__: (<class  object >,)
Employee.__dict__: {
    displayCount : <function Employee.displayCount at 0x0160D2B8>, 
    __module__ :  __main__ ,  __doc__ :  Common base class for all employees , 
    empCount : 2,  __init__ : 
   <function Employee.__init__ at 0x0124F810>,  displayEmployee : 
   <function Employee.displayEmployee at 0x0160D300>,
    __weakref__ : 
   <attribute  __weakref__  of  Employee  objects>,  __dict__ : 
   <attribute  __dict__  of  Employee  objects>
}

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/python3

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

140338326963984 140338326963984 140338326963984
Point destroyed

#!/usr/bin/python3
import point

p1 = point.Point()

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

#!/usr/bin/python3

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 calss B
.....

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

#!/usr/bin/python3

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/python3

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/python3

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