Classes and Objects

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.

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 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.

A very basic class would look something like this:

#!/usr/bin/python

class MyClass:
    variable = "blah"

    def function(self):
        print "This is a message inside the function in class."

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

We'll explain why you have to include that "self" as a parameter a little bit later. First, to assign the above class(template) to an object you would do the following:

myobjectx = MyClass()

Now the variable "myobjectx" holds an object of the class "MyClass" that contains the variable and the function defined within the class called "MyClass".

Accessing Object Variables

To access the variable inside of the newly created object "myobjectx" you would do the following:

myobjectx.variable

So for instance the below would output the string "blah":

print myobjectx.variable

You can create multiple different objects that are of the same class(have the same variables and functions defined). However, each object contains independent copies of the variables defined in the class.

For instance, if we were to define another object with the "MyClass" class and then change the string in the variable above:

myobjecty = MyClass()

myobjecty.variable = "HelloRocky"

Then print out both values:

print myobjectx.variable   # This would print "blah".
print myobjecty.variable   # This would print "HelloRocky".

Result

blah
HelloRocky

Accessing Object Functions

To access a function inside of an object you use notation similar to accessing a variable:

myobjectx.function()

The above would print out the message :

This is a message inside the function in class.

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