Class methods and cls usage in Python - Mini Project: Build & Apply

Class methods and cls usage

📖 Scenario: Imagine you are creating a simple system to track how many books have been added to a library. Each book has a title and an author. You want to keep count of all books created using a class method.

🎯 Goal: You will build a Book class that keeps track of the total number of books created using a class method and the cls keyword.

📋 What You'll Learn

Create a class called Book with attributes title and author

Add a class variable count to track the number of books

Create a class method called increment_count that increases count by 1 using cls

Use the class method inside the constructor to update the count each time a new book is created

Print the total number of books using the class variable

💡 Why This Matters

🌍 Real World

Tracking counts of objects like books, users, or orders is common in software. Class methods help manage data shared across all objects.

💼 Career

Understanding class methods and class variables is important for writing clean, organized code in many programming jobs, especially in object-oriented programming.

1

Create the Book class with attributes

Create a class called Book with an __init__ method that takes self, title, and author as parameters. Inside __init__, set self.title to title and self.author to author.

Python

class Book:
    def __init__(self, title, author):
        # Your code here

Need a hint?

Remember to use self to set instance variables inside __init__.

2

Add a class variable and class method

Add a class variable called count to Book and set it to 0. Then create a class method called increment_count using the @classmethod decorator. It should take cls as a parameter and increase cls.count by 1.

Python

class Book:
    count = 0
    def __init__(self, title, author):
        self.title = title
        self.author = author
    
    @classmethod
    def increment_count(cls):
        # Your code here

Need a hint?

Use @classmethod above the method and cls.count += 1 inside it.

3

Call the class method inside the constructor

Inside the __init__ method, call the class method increment_count using Book.increment_count() to update the book count whenever a new book is created.

Python

class Book:
    count = 0
    def __init__(self, title, author):
        self.title = title
        self.author = author
        # Your code here
    
    @classmethod
    def increment_count(cls):
        cls.count += 1

Need a hint?

Call the class method inside __init__ using the class name Book.increment_count().

4

Create books and print total count

Create three Book objects with any titles and authors you like. Then print the total number of books by printing Book.count.

Python

class Book:
    count = 0
    def __init__(self, title, author):
        self.title = title
        self.author = author
        Book.increment_count()
    
    @classmethod
    def increment_count(cls):
        cls.count += 1

# Your code here

Need a hint?

Create three books and print Book.count to see the total.