Inheritance and init method in Python

I'm begginer of python. I can't understand inheritance and __init__().

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

开发者_运维知识库class Num2(Num):
    def show(self):
        print self.n1

mynumber = Num2(8)
mynumber.show()

RESULT: 8

This is OK. But I replace Num2 with

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

RESULT: Error. Num2 has no attribute "n1".

In this case, how can Num2 access n1?

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

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

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()

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