Object-Oriented Programming in Python

Object-Oriented Programming (OOP) is a programming paradigm based on the concept of “objects” — containers that bundle data and behavior together. Python, while supporting multiple paradigms, is an object-oriented language at its core. Understanding OOP is essential for writing structured, maintainable, and scalable code.

In this chapter, we will explore OOP fundamentals in Python: classes, objects, attributes, methods, constructors, inheritance, polymorphism, encapsulation, abstraction, and real-world design patterns. You’ll also learn best practices, common pitfalls, and how to structure large applications using OOP.

1. What is Object-Oriented Programming?

OOP organizes code around objects and classes instead of functions and logic.

Key Concepts:

• Class: Blueprint for creating objects
• Object: Instance of a class
• Attribute: Data associated with an object
• Method: Function associated with an object

Real-world Analogy:

A class is like a car blueprint, and an object is the actual car built from that blueprint.

2. Creating a Class

Syntax:

class ClassName:
    # class body

Example:

class Dog:
    def bark(self):
        print("Woof!")

3. Creating Objects (Instances)

my_dog = Dog()
my_dog.bark()  # Output: Woof!

Each object has its own identity and can access class methods.

4. The __init__() Method (Constructor)

Special method that runs automatically when an object is created.

class Dog:
    def __init__(self, name, age):
        self.name = name
        self.age = age

Creating Objects:

d = Dog("Buddy", 3)
print(d.name, d.age)

5. Instance and Class Attributes

Instance Attributes:

Defined using self inside the __init__() method.

Class Attributes:

Defined outside all methods, shared among all instances.

class Dog:
    species = "Canine"

6. Instance Methods

Most common type, defined with self as the first parameter.

class Dog:
    def speak(self):
        print(f"My name is {self.name}")

7. Class Methods and Static Methods

Class Methods:

Defined with @classmethod, receive class (cls) instead of instance.

@classmethod
def from_string(cls, data):
    name, age = data.split("-")
    return cls(name, int(age))

Static Methods:

Defined with @staticmethod, behave like regular functions but belong to the class namespace.

@staticmethod
def greet():
    print("Welcome!")

8. Encapsulation

Protecting internal object data by making attributes private or protected.

class Account:
    def __init__(self, balance):
        self.__balance = balance  # private

    def get_balance(self):
        return self.__balance

9. Inheritance

Allows a class to inherit attributes and methods from another class.

Syntax:

class ChildClass(ParentClass):
    # additional members

Example:

class Animal:
    def speak(self):
        print("Animal sound")

class Cat(Animal):
    def speak(self):
        print("Meow")

super() Function:

Calls parent class method.

class Dog(Animal):
    def __init__(self):
        super().__init__()

10. Polymorphism

Ability to use a common interface for different data types.

class Bird:
    def speak(self):
        print("Chirp")

class Human:
    def speak(self):
        print("Hello")

for entity in [Bird(), Human()]:
    entity.speak()  # Different outputs

11. Abstraction

Hiding implementation details and showing only the interface.

Abstract Base Classes:

Use abc module.

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

12. Special Methods (Dunder Methods)

Double underscore (dunder) methods customize class behavior.

Method	Purpose
__str__	String representation
__repr__	Official representation
__len__	Length of object
__eq__	Equality comparison
__add__	Overload + operator

Example:

class Book:
    def __init__(self, title):
        self.title = title

    def __str__(self):
        return self.title

13. Composition vs Inheritance

Composition:

Class contains other class objects.

class Engine:
    pass

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

Use composition when a “has-a” relationship makes more sense than “is-a”.

14. Real-World Example: Employee Management System

class Employee:
    def __init__(self, name, id):
        self.name = name
        self.id = id

    def show(self):
        print(f"Name: {self.name}, ID: {self.id}")

class Manager(Employee):
    def __init__(self, name, id, department):
        super().__init__(name, id)
        self.department = department

    def show(self):
        super().show()
        print(f"Department: {self.department}")

15. Best Practices

• Use self consistently
• Favor composition over deep inheritance chains
• Encapsulate data properly
• Use class methods for alternative constructors
• Use __str__() for meaningful object representation
• Write unit tests for class methods

16. Common Mistakes

• Forgetting self in instance methods
• Overriding methods without calling super()
• Using mutable default arguments in __init__
• Confusing class and instance variables

Debugging Tips:

• Use type() and dir() to inspect objects
• Add print() statements to trace method calls

17. Exercises

1. Create a class Rectangle with width, height and area methods.
2. Implement inheritance with Vehicle and Car classes.
3. Use @classmethod to instantiate an object from a string.
4. Implement a BankAccount class with encapsulated balance.
5. Create a polymorphic function speak() used by Dog, Cat, and Bird classes.
6. Use the abc module to define and implement an abstract base class.

18. Summary

Object-Oriented Programming is a cornerstone of software development. With classes, objects, encapsulation, inheritance, polymorphism, and abstraction, Python allows developers to model real-world entities in code. Mastering OOP leads to cleaner, more maintainable, and reusable code — a vital skill for any serious Python developer.

✅ Next Chapter: Modules and Packages – Learn how to organize your Python codebase using modular and scalable structures.