Method Resolution Order in Python Inheritance

Last Updated : 15 Jan, 2026

Method Resolution Order (MRO) defines the order in which Python searches for a method in a class and its parent classes. It becomes important when the same method exists in more than one class in an inheritance chain, especially in multiple inheritance.

he example shows how Python decides which method to execute when both a parent and a child class have a method with the same name.

Python 
class A:
    def fun(self):
        print("In class A")

class B(A):
    def fun(self):
        print("In class B")

a = B()
a.fun()

Output
In class B

Explanation:

  • When obj.fun() is called, Python first looks in class B.
  • Since B defines fun(), it runs that method and does not check class A.
  • The MRO here is: B -> A.

Multiple Inheritance (Diamond Problem)

In multiple inheritance, a child class can inherit from more than one parent class. When these parent classes come from a common base class, the structure forms a diamond shape and Python must decide which class method should be used.

Python 
class A:
    def fun(self):
        print("In class A")

class B(A):
    def fun(self):
        print("In class B")

class C(A):
    def fun(self):
        print("In class C")

class D(B, C):
    pass

a = D()
a.fun()

Output
In class B

Explanation:

  • D inherits from B and C.
  • Python follows the MRO: D -> B -> C -> A.
  • Since B has fun(), it is executed and the search stops.

C3 Linearization in MRO

Python uses the C3 linearization algorithm to decide the order in which classes are searched when a method is called. This algorithm produces a single, consistent order that respects both inheritance and the order in which parent classes are written.

Example: This program prints the exact order Python uses for method lookup.

Python 
class A:
    def fun(self):
        print("In class A")

class B(A):
    def fun(self):
        print("In class B")

class C(A):
    def fun(self):
        print("In class C")

class D(B, C):
    pass

obj = D()
obj.fun()

print(D.__mro__)

Output
In class B
(<class '__main__.D'>, <class '__main__.B'>, <class '__main__.C'>, <class '__main__.A'>, <class 'object'>)

Explanation:

  • When obj.fun() is called, Python follows the MRO shown by D.__mro__.
  • It checks D, then B, then C, then A.
  • Since B defines fun(), that method is executed and the search stops.

Methods to View Method Resolution Order (MRO) of a Class

Python provides two ways to check the method resolution order (MRO) of a class:

  • __mro__ attribute: shows a tuple of classes in the order Python searches for methods.
  • mro() method: returns a list of classes in the MRO.
Python 
class A:
    def fun(self):
        print("In class A")

class B:
    def fun(self):
        print("In class B")

class C(A, B):
    def __init__(self):
        print("Constructor C")

obj = C()

print(C.__mro__) 
print(C.mro())

Output
Constructor C
(<class '__main__.C'>, <class '__main__.A'>, <class '__main__.B'>, <class 'object'>)
[<class '__main__.C'>, <class '__main__.A'>, <class '__main__.B'>, <class 'object'>]

Explanation:

  • C.__mro__ returns the method resolution order as a tuple.
  • C.mro() returns the same order as a list.
  • This sequence shows that Python will search C, then A, then B, and finally object when resolving any method call.
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