Inheritance and MRO

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Inheritance is one of the core pillars of Object-Oriented Programming (OOP) in Python. It allows a new class (child) to reuse, extend, and customize the behavior of an existing class (parent), promoting code reusability and logical organization.

Python supports multiple forms of inheritance and uses a well-defined mechanism called Method Resolution Order (MRO) to decide which method is executed when multiple classes define methods with the same name.

What is Inheritance?

• Inheritance allows a class to acquire methods and properties of another class
• The existing class is called the parent (base) class
• The new class is called the child (derived) class
• Child classes can access parent methods, but parents cannot access child methods

Types of Inheritance in Python

class A:
    def f1(self):
        print("f1 works")

    def f2(self):
        print("f2 works")

class B(A):
    def f3(self):
        print("f3 works")

    def f4(self):
        print("f4 works")

obj = B()
obj.f1()

f1 works

Access Rules in Inheritance

• A child class can access all parent methods
• A parent class cannot access child methods
• Sibling classes cannot access each other’s methods

Example

class C(A):
    def f5(self):
        print("f5 works")

obj = B()
obj.f5()

Output

AttributeError: 'B' object has no attribute 'f5'

class A:
    def f1(self):
        print("f1 works")

    def f2(self):
        print("f2 works")

class B(A):
    def f3(self):
        print("f3 works")

    def f4(self):
        print("f4 works")

class C(A):
    def f5(self):
        print("f5 works")

obj = B()
obj.f4()

f4 works

class A:
    def show(self):
        print("in A show")

class B:
    def show(self):
        print("in B show")

class C(A, B):
    pass

obj = C()
obj.show()

in A show

Method Resolution Order (MRO)

What is MRO?

• MRO defines the order in which Python searches classes for a method

• Python searches:

  1. The child class
  2. Parent classes from left to right
  3. Grandparents (if any)

MRO Depends on Inheritance Order

Example

class C(B, A):
    pass

obj = C()
obj.show()

Output

in B show

➡ Changing the order of parent classes changes the method selected.

Explicit Parent Method Call

If needed, you can directly call a specific parent’s method.

B.show(obj)

Output

in B show

This bypasses MRO and calls the method explicitly.

How Python Resolves Method Conflicts?

Child class methods take highest priority, if not found, Python checks parent classes in MRO order

Summary

• Inheritance is a powerful feature in Python that enables structured code reuse and extensibility.
• Child classes can access parent methods, not vice versa.
• Python supports single-level, multi-level, and multiple inheritance.
• When multiple inheritance is used, Python relies on Method Resolution Order (MRO) to determine which method to execute, ensuring predictable and consistent behavior in complex class hierarchies.