Method Resolution Order

There are multiple types of inheritance:

1. Simple Inheritance
2. Multiple Inheritance
3. Multilevel Inheritance
4. Hierarchical Inheritance
5. Hybrid Inheritance

MRO determines the Order in which a given method or attribute passed is searched.

Object is first searched in class Object then the parent class:

But what about multiple inheritance? Here, first the method is searched in the child class then the class Y and X.

Old version of python used to use Depth First Search DFS algorithm. But from python 3.0 it uses C3 linearization

Some features of C3 linearization algorithm’s are:

Following is an example of MRO attribute: Here the num=9 from the class B will be inherited.

Help function provides the MRO on top and is much more descriptive:

class A:
    def d(self):
        return "Function inside A"
 
 
class B:
    def d(self):
        return "Function inside B"
 
 
class C:
    def d(self):
        return "Function inside C"
 
 
class D(A, B):
    def d(self):
        return "Function inside D"
 
 
class E(B, C):
    def d(self):
        return "Function inside E"
 
 
class F(E, D, C):
    pass
 
 
f = F()
print(f.d())
print(F.mro())

This outputs:

Function inside E
[<class '__main__.F'>, <class '__main__.E'>, <class '__main__.D'>, <class '__main__.A'>, <class '__main__.B'>, <class '__main__.C'>, <class 'object'>]

Why A comes after D?