Objects and Class

It’s time to learn about core functionality of Python programming language and that is Python objects and classes. In this lesson you’ll learn how to create it and use it in your Python program. As mentioned many times before Python is an object oriented programming language just like C++ and many others. Just to clarify the difference between procedural and object oriented programming the emphasis in procedural programming is on functions while in object oriented programming the main focus are objects.

Collection of data (variables) and methods (functions) that act on those data are what make objects. We can think of a class as a sketch or prototype. Class is like a sketch of a car because it contains all the details about tires, engine, lights, seats … Based on this description a car is built and the car is an object.

So if the class is a blueprint for building a car we can conclude that we can create many objects from this class. You will find in literature that object is an instance of a class and the process of creating this object is called instantiation.

How to Create a Class? 

Now let’s start with something simple. Let’s create a simple class in Python. To define a class use the keyword class. After the class, give a name of the class and the first line after class declaration is usually used for a brief description about the class. This line (string) is called docstring and while it’s not mandatory it is usually recommended (just to know why you created this class).

class myFirstclass:

      "This line is a docstring and it usualy say something about a class"

      Pass

A class creates a new local namespace where all its attributes are defined. Attributes may be data or functions. There are special attributes that can be used and they begin with double underscore ‘__’. An example of that attribute is __doc__ and it gives us the docstring of that class.

>>> class myFirstclass: ...       "This line is a docstring and it usualy say something about a class" ...       pass ... >>> myFirstclass.__doc__

'This line is a docstring and it usualy say something about a class'

When we create a class, a new class object is created with the same name. This class object allows us to access the different attributes as well as to instantiate new objects of that class.

class mySecondclass:

    "This is my second class"

    a = 50

    def function(self):

        print('Hello')

print 'a = ' + str (mySecondclass.a)

a = 50

print mySecondclass.function

print "DOCSTRING = " + str(mySecondclass.__doc__)

DOCSTRING = This is my second class

 How to Create an Object? 

We saw that the class object could be used to access different attributes. It can also be used to create new object instances (instantation) of that class. The procedure of creating an object is similar to a function call.

obj = mySecondclass()

This command will create a new instance of an object named obj. We can access attributes of objects using the object name as prefix.

class mySecondclass:

    "This is my second class"

    a = 50

    def function(self):

        print('Hello')

obj = mySecondclass()

"

"

Self - parameter is parameter that will always be used as reference back to the object itself. The object itself is passed as first argument. So obj.function() translates to mySecondclass.func(obj). In general calling a method with a list of n arguments is equivalent to calling the corresponding function with an argument list that is created by inserting methods object before the first argument. For that reason the first argument of the function in the class must be object itself.