Python Inheritance & Polymorphism: Types, super() & Method Resolution Order

Inheritance is a cornerstone of Object-Oriented Programming that establishes an "is-a" relationship between classes. A Derived (Child) Class inherits attributes and methods from a Base (Parent) Class, enabling code reusability, modular hierarchy, and polymorphism.

Figure 32.1: The 5 types of inheritance supported in Python: Single, Multiple, Multilevel, Hierarchical, and Hybrid.

1. Single Inheritance & Method Overriding with super()

In single inheritance, a derived class inherits from exactly one parent class:

class Person:
    def __init__(self, name: str, age: int):
        self.name = name
        self.age = age

    def introduce(self):
        print(f"Hi, I am {self.name}, age {self.age}.")

# Derived Class inheriting from Person:
class Student(Person):
    def __init__(self, name: str, age: int, roll_no: int, major: str):
        # Call parent constructor using super():
        super().__init__(name, age)
        self.roll_no = roll_no
        self.major = major

    # Method Overriding:
    def introduce(self):
        super().introduce()  # Call parent method
        print(f"I am a {self.major} student (Roll: {self.roll_no}).")

# Usage:
s = Student("Rohan", 21, 109, "BCA")
s.introduce()

2. Multiple Inheritance & The Diamond Problem

Python allows a child class to inherit from multiple parents simultaneously:

class Flyable:
    def move(self):
        print("I am flying through the air!")

class Swimmable:
    def move(self):
        print("I am swimming across the river!")

# Inherits from both parents:
class Duck(Flyable, Swimmable):
    pass

d = Duck()
d.move()  # Which move() is called? Flying or Swimming?

3. Method Resolution Order (MRO) & C3 Linearization

To resolve the order in which inherited methods are searched, Python uses the C3 Linearization Algorithm. You can inspect the MRO of any class using Class.mro() or Class.__mro__:

print("Duck MRO:", Duck.mro())
# Output: [Duck, Flyable, Swimmable, object]
# Because Flyable appears first in Duck(Flyable, Swimmable), its move() is executed!

Frequently Asked Questions (FAQ)