Class inheritance

Class inheritance is an important subject to understand when it comes to objected oriented programming. It is the basis in which we can code as little as possible yet still have it contain as much information as possible. I want to focus on the initialization aspect of class inheritance and step through it so it becomes more clear.

Consider this simple class example at first:

class A():
    def __init__(self):
        print('Initialize A')


a_obj = A()

You will see that the moment a_obj is called, it initialized the class A which will print

>>> Initialize A

So far so good!

Now we will consider creating a class B that will inherit from class A and go from there.

class B(A):
    def __init__(self):
        A.__init__(self)
        print('Initialize B')

What is happening here is class B(A) is telling us that this class B is going to inherit from class A. But just that by itself does not do much. Typically, a well written python module would separate class and instance-level stuff by declaring them at the appropriate places. Instance variables, for example, would typically be delcared inside a class object’s __init__ method. Class variables would be declared outside the scope of the __init__ method. As such, you must initiate the class A inside class B in order to be able to use all the stuff that’s initiated inside class A. That is where A.__init_(self) comes in handy.

Now, if you step through this:

class A():
    def __init__(self):
        print('Initialize A')


class B(A):
    def __init__(self):
        A.__init__(self)
        print('Initialize B')


b_obj = B()

We will step through this process now step-by-step.

b_obj = B() #We begin by initiating the class B via b_obj object.

class B(A): #b_obj will bring us to class B which inherits from A.

def __init__(self): #B.__init__() will be initiated.

A.__init__(self) #Now this will initialize class A.

def __init__(self): #We enter A.__init__()

print('Initialize A') #This print statement will now be executed and exit A.__init__().

print('Initialize B') #This print statement will now be executed and exit B.__init__().

So ultimately what you will see as a result of the code blocks above is:

>>> Initialize A
>>> Initialize B

As you can see, A.__init__(self) is key to initializing class A. It also means that you can place that statement not necessarily immediately after B’s __init__ if you so choose.