CODEWITHSUNDEEP
pythoninheritanceinit
Yugo Kamo
Yugo Kamo

Reputation: 2389

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

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?

Upvotes: 132

Views: 193858

Answers (5)

user8395964
user8395964

Reputation: 144

Based on this: Can I collect and merge different answers into an accepted one?

I am creating a wiki answer collecting input from above two latest answers:
Answer with code by Sacchit Jaiswal & Tom Jelen
Answer with reason or why it's needed by Mauro Rocco

When you override the init you have also to call the init of the parent class
Understanding Python super() with __init__() methods

super(Num2, self).__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()

Upvotes: 0

Sacchit Jaiswal
Sacchit Jaiswal

Reputation: 281

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()

Upvotes: 28

Mauro Rocco
Mauro Rocco

Reputation: 5128

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

Upvotes: 47

Danny Milosavljevic
Danny Milosavljevic

Reputation: 642

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

Upvotes: 4

Mario Duarte
Mario Duarte

Reputation: 3235

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

Upvotes: 198

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