Multiple inheritance | Python

Question

Why A is not called in this code: [is it because of mro : left to right then A class should be called?]

class A:
    
     def __init__(self,name):
        print('inside a',name)
        
class B:
    
     def __init__(self,name):
        print('inside b',name)
        
class C(B,A):
    
     def __init__(self,name):
        print('inside c',name)
        super().__init__(name)
        
c = C('hello')

Output :

inside c hello

inside b hello

But when I defined it like this basically a parent class then it is working properly as expected.[Why A class is called here ]Code:

class D:
    
    def __init__(self,name):
        print('inside d',name)
    
class A(D):
    
     def __init__(self,name):
        print('inside a',name)
        super().__init__(name)
        
class B(D):
    
     def __init__(self,name):
        print('inside b',name)
        super().__init__(name)
        
class C(B,A):
    
    def __init__(self,name):
        print('inside c',name)
        super().__init__(name)
        
c = C('hello')

output:

inside c hello

inside b hello

inside a hello

inside d hello

For super() to work right with multiple inheritance, you have to use it in every class in the hierarchy - even the base classes. — jasonharper
– jasonharper, Commented Aug 30, 2020 at 13:48

Tomato Master · Accepted Answer · 2020-08-31 09:48:59Z

2

As per Method resolution order, the members are searched in depth first search methodology i.e in your first example :

class A:
    
     def __init__(self,name):
        print('inside a',name)
class B:
    
     def __init__(self,name):
        print('inside b',name)       


class C(B,A):
    
     def __init__(self,name):
        print('inside c',name)
        super().__init__(name)
        
c = C('hello')

First: constructor of C is called.

Second: Since you have super().init(name) in you class C , it will call its left parent i.e B.

Third: it will try to go to right(class C) but since you have not written super().init(name) inside class B ,the constructor of class A can't be called because from class B it can't move to Object class

If you will write super().init(name) in class B , it will iterate from Object class to right of object i.e class A

For e.g.:

class A:
    
     def __init__(self,name):
        print('inside a',name)
        
class B:

     def __init__(self,name):
        print('inside b',name)
        super().__init__(name)

        
class C(B,A):
    
     def __init__(self,name):
        print('inside c',name)
        super().__init__(name)

        
c = C('hello')

For more visit :https://www.geeksforgeeks.org/method-resolution-order-in-python-inheritance/

answered Aug 31, 2020 at 9:48

Tomato Master

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1 Comment

Tomato Master Over a year ago

You can also view C3 linearization, to understand MRO better en.wikipedia.org/wiki/…

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