Inheritance and init method in Python

In the first situation, Num2 is extending the class Num and since you are not redefining the special method named __init__() in Num2, it gets inherited from Num.

When a class defines an __init__() method, class instantiation automatically invokes __init__() for the newly-created class instance.

In the second situation, since you are redefining __init__() in Num2 you need to explicitly call the one in the super class (Num) if you want to extend its behavior.

class Num2(Num):
    def __init__(self,num):
        Num.__init__(self,num)
        self.n2 = num*2

A simple change in Num2 class like this:

super().__init__(num) 

It works in python3.

class Num:
        def __init__(self,num):
                self.n1 = num

class Num2(Num):
        def __init__(self,num):
                super().__init__(num)
                self.n2 = num*2
        def show(self):
                print (self.n1,self.n2)

mynumber = Num2(8)
mynumber.show()

When you override the init you have also to call the init of the parent class

super(Num2, self).__init__(num)

Understanding Python super() with __init__() methods

Since you don't call Num.__init__ , the field "n1" never gets created. Call it and then it will be there.