Chapter 10: OOP in Python – Classes & Objects

1. OOP Concepts

Classes are blueprints, objects are instances

Class

A blueprint for creating objects

class Dog:
    pass

Object

An instance of a class

my_dog = Dog()

Attribute

Data associated with an object

my_dog.name = "Rover"

Method

Function associated with an object

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

2. Defining Classes

Basic Class Structure

class Car:
    # Class attribute (shared by all instances)
    wheels = 4
    
    # Constructor (initialize instance attributes)
    def __init__(self, brand, model):
        self.brand = brand  # Instance attribute
        self.model = model
    
    # Instance method
    def description(self):
        return f"{self.brand} {self.model}"

# Create objects
my_car = Car("Toyota", "Corolla")
print(my_car.description())  # Toyota Corolla

Exercise 1: Create a Book Class

Create a Book class with:

Attributes: title, author, pages
Method info() that returns a description
Create an instance and call the method

class Book:
    def __init__(self, title, author, pages):
        self.title = title
        self.author = author
        self.pages = pages
    
    def info(self):
        return f"{self.title} by {self.author}, {self.pages} pages"

book = Book("Python 101", "John Doe", 250)
print(book.info())  # Python 101 by John Doe, 250 pages

3. Inheritance

Base (Parent) Class

class Animal:
    def __init__(self, name):
        self.name = name
    
    def speak(self):
        raise NotImplementedError("Subclass must implement")

Derived (Child) Class

class Dog(Animal):
    def speak(self):
        return f"{self.name} says Woof!"

class Cat(Animal):
    def speak(self):
        return f"{self.name} says Meow!"

Usage

dog = Dog("Buddy")
cat = Cat("Whiskers")
print(dog.speak())  # Buddy says Woof!
print(cat.speak())  # Whiskers says Meow!

Exercise 2: Shape Inheritance

Create a base Shape class with:

Method area() that raises NotImplementedError
Derived classes Rectangle and Circle
Implement area() in each subclass

import math

class Shape:
    def area(self):
        raise NotImplementedError("Subclass must implement")

class Rectangle(Shape):
    def __init__(self, width, height):
        self.width = width
        self.height = height
    
    def area(self):
        return self.width * self.height

class Circle(Shape):
    def __init__(self, radius):
        self.radius = radius
    
    def area(self):
        return math.pi * self.radius ** 2

rect = Rectangle(5, 4)
print(rect.area())  # 20

circle = Circle(3)
print(circle.area())  # ~28.27

4. Encapsulation

Private Attributes (Name Mangling)

class BankAccount:
    def __init__(self, balance):
        self.__balance = balance  # Private attribute
    
    def deposit(self, amount):
        self.__balance += amount
    
    def get_balance(self):
        return self.__balance

account = BankAccount(1000)
account.deposit(500)
print(account.get_balance())  # 1500
# account.__balance  # Error: AttributeError

Python doesn't have truly private attributes, but name mangling (__attribute) makes them harder to access accidentally.

5. Polymorphism

class Dog:
    def speak(self):
        return "Woof!"

class Cat:
    def speak(self):
        return "Meow!"

def animal_sound(animal):
    print(animal.speak())

dog = Dog()
cat = Cat()

animal_sound(dog)  # Woof!
animal_sound(cat)  # Meow!

Exercise 3: Polymorphic Payment

Create payment classes that demonstrate polymorphism:

Base class Payment with process() method
Subclasses CreditCard, PayPal, BankTransfer
Each implements process() differently

class Payment:
    def process(self):
        raise NotImplementedError

class CreditCard(Payment):
    def process(self):
        return "Processing credit card payment"

class PayPal(Payment):
    def process(self):
        return "Processing PayPal payment"

class BankTransfer(Payment):
    def process(self):
        return "Processing bank transfer"

payments = [CreditCard(), PayPal(), BankTransfer()]
for payment in payments:
    print(payment.process())

Previous Chapter Next Chapter