Composition vs Inheritance in Python: When to Use Which

Table of Contents

• Introduction
• What is Inheritance?
• What is Composition?
• Inheritance vs Composition: Core Differences
• When to Use Inheritance
• When to Use Composition
• Practical Examples: Inheritance vs Composition
• Benefits and Drawbacks of Each
• Best Practices for Designing Class Relationships
• Conclusion

Introduction

When designing object-oriented systems, developers often face a critical architectural decision: should I use inheritance or composition?
Both are key principles of OOP (Object-Oriented Programming) and allow us to build complex applications. However, the choice between them can significantly impact your system’s scalability, flexibility, and maintainability.

This article explores both inheritance and composition deeply, helps you understand the differences, and provides guidelines on when and why to use each approach.

What is Inheritance?

Inheritance allows a class (called the child class or subclass) to inherit attributes and methods from another class (called the parent class or superclass).

Example:

class Animal:
    def move(self):
        print("Moving...")

class Dog(Animal):
    def bark(self):
        print("Barking...")

dog = Dog()
dog.move()
dog.bark()

The Dog class inherits the move() method from Animal.
Inheritance expresses an “is-a” relationship:
Dog is a type of Animal.

What is Composition?

Composition means that one class contains an instance of another class and delegates work to it. Rather than being a subclass, a class uses another class.

Example:

class Engine:
    def start(self):
        print("Engine started")

class Car:
    def __init__(self):
        self.engine = Engine()

    def drive(self):
        self.engine.start()
        print("Car is moving")

car = Car()
car.drive()

Here, the Car class has-a Engine.
This is a “has-a” relationship:
Car has an Engine.

Inheritance vs Composition: Core Differences

When to Use Inheritance

Use inheritance when:

• You are modeling a true “is-a” relationship.
• Subclasses can share and extend functionality naturally.
• The base class is stable and unlikely to change frequently.
• You want to enable polymorphism where different subclasses override parent behavior.

Example:

• A Rectangle is a Shape.
• A Cat is an Animal.

Important: Inheritance is best when used carefully and sparingly. Deep hierarchies should be avoided.

When to Use Composition

Use composition when:

• You need to reuse code across different, unrelated classes.
• Your system requires flexibility to swap components.
• You are building complex objects that are better represented by combining smaller objects.
• The relationships between classes are better modeled as “has-a” rather than “is-a”.

Example:

• A Car has an Engine.
• A Printer has a PaperTray.

Composition leads to low coupling and high cohesion, which makes systems more maintainable.

Practical Examples: Inheritance vs Composition

Inheritance Example:

class Bird:
    def fly(self):
        print("Flying...")

class Sparrow(Bird):
    def chirp(self):
        print("Chirping...")

sparrow = Sparrow()
sparrow.fly()
sparrow.chirp()

Here, Sparrow is a natural type of Bird.

Composition Example:

class Battery:
    def charge(self):
        print("Charging battery")

class ElectricCar:
    def __init__(self):
        self.battery = Battery()

    def drive(self):
        self.battery.charge()
        print("Driving electric car")

tesla = ElectricCar()
tesla.drive()

Here, ElectricCar is not a type of Battery; it has a battery.

Benefits and Drawbacks of Each

Benefits of Inheritance:

• Encourages code reuse via subclassing.
• Enables polymorphism (objects of different classes can be treated uniformly).
• Helps express hierarchical relationships.

Drawbacks of Inheritance:

• Tight coupling between base and derived classes.
• Changes to a parent class can unexpectedly break child classes.
• Deep inheritance trees are hard to understand and maintain.

Benefits of Composition:

• Loosely coupled components promote flexibility.
• Easy to change or replace parts of a system.
• Encourages smaller, more focused classes (high cohesion).
• Facilitates testing and maintenance.

Drawbacks of Composition:

• Might require more upfront design effort.
• Can introduce indirection layers (one object delegates to another), making debugging harder if not carefully managed.

Best Practices for Designing Class Relationships

• Favor Composition over Inheritance: Especially when designing for flexibility and future growth.
• Use Inheritance Only for True Hierarchies: Only when there’s a clear “is-a” relationship.
• Keep Inheritance Hierarchies Shallow: Prefer flatter structures for easier maintainability.
• Design for Change: Assume that change is inevitable and structure your code to minimize ripple effects.
• Follow SOLID Principles: Particularly the Liskov Substitution Principle (subtypes must be replaceable with their base types without altering correctness).

Conclusion

Both inheritance and composition are essential tools for building object-oriented systems in Python.
Inheritance is suitable when modeling natural hierarchies and promoting code reuse through specialization.
Composition is often a better choice for building flexible, modular, and maintainable systems.

A good Python developer understands not just how to use these techniques, but when and why to choose one over the other.
Always prefer clarity, maintainability, and scalability in your design decisions.