Concept of classes in python !

In all our python programs till now, we have designed our program around functions or blocks of statements which manipulate data. This is called the procedure-oriented way of programming. There is another way of organizing your program which is to combine data and functionality and wrap it inside what is calledan object. This is called the object oriented  programming paradigm.

Before move to advance, let's 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 aren't 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 (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 individual 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.

Python is fully object-oriented: you can define your own classes, inherit from your own or built in classes, and instantiate the classes you've defined.The self Class methods have only one specific difference from ordinary functions - they must have an extra first name that has to be added to the beginning of the parameter list, but you do do not give a value for this parameter when you call the method, Python will provide it. This particular variable refers to the object itself, and by convention, it is given the name self.Although, you can give any name for this parameter, it is strongly recommended that you use the name self - any other name is definitely frowned upon.

Creating a Class :
class Person:
pass # An empty block

We create a new class using the class statement followed by the name of the class. This follows an indented block of statements which form the body of the class. In this case, we have an empty block which is indicated using the pass statement.

Using Object Methds :
class Person:
    def sayHi(self):
        print 'Hello world'

                           

      

The __init__ method :
The method is useful to do any initialization you want to do with your object. Note that The __init__() method can accept parameters, but does not return anything. It sets the internal state of the object.

class Person:
    def __init__(self, name):
        self.name = name
    def sayHi(self):
        print 'Hello, my name is', self.name


we define the __init__ method as taking a parameter name (along with the usual self). Here,we just create a new field also called name. Notice these are two different variables even though they have the same name. The dotted notation allows us to differentiate between them.
Most importantly, notice that we do not explicitly call the __init__ method but pass the arguments in the parentheses following the class name when creating a new instance of the class. This is the special significance of this method.

Built-In Class Attributes:

Every Python class keeps following built-in attributes and they can be accessed using dot operator like any other attribute:

    __dict__ : Dictionary containing the class's namespace.

    __doc__ : Class documentation string or None if undefined.

    __name__: Class name.

    __module__: Module name in which the class is defined. This attribute is "__main__" in interactive mode.

    __bases__ : A possibly empty tuple containing the base classes, in the order of their occurrence in the base class list.