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