Class Method Decorator

Part of the Object Oriented Programming section of Coddy's Python journey — lesson 12 of 64.

The @classmethod decorator creates methods that receive the class itself as the first parameter (cls) instead of an instance (self).

Here is an example of a class with class methods:

class Student:
    school_name = "Python Academy"
    student_count = 0
    
    def __init__(self, name):
        self.name = name
        Student.student_count += 1
    
    @classmethod
    def get_school_info(cls):
        return f"School: {cls.school_name}, Students: {cls.student_count}"
    
    @classmethod
    def create_guest_student(cls):
        return cls("Guest")

Call class methods using the class name:

info = Student.get_school_info()
print(info)

Create some students:

alice = Student("Alice")
bob = Student("Bob")

updated_info = Student.get_school_info()
print(updated_info)

Use class methods as alternative constructors:

guest = Student.create_guest_student()
print(guest.name)
print(Student.get_school_info())

Output:

School: Python Academy, Students: 0
School: Python Academy, Students: 2
Guest
School: Python Academy, Students: 3

Class methods can also be called from instances:

alice = Student("Alice")
info_from_instance = alice.get_school_info()
print(info_from_instance)

Output:

School: Python Academy, Students: 1

Key Point: Use @classmethod when you need to access class attributes or create alternative ways to construct objects. The first parameter is cls (the class itself), not self.

Cheat sheet

The @classmethod decorator creates methods that receive the class itself as the first parameter (cls) instead of an instance (self).

class Student:
    school_name = "Python Academy"
    student_count = 0
    
    def __init__(self, name):
        self.name = name
        Student.student_count += 1
    
    @classmethod
    def get_school_info(cls):
        return f"School: {cls.school_name}, Students: {cls.student_count}"
    
    @classmethod
    def create_guest_student(cls):
        return cls("Guest")

Call class methods using the class name:

info = Student.get_school_info()
guest = Student.create_guest_student()

Class methods can also be called from instances:

alice = Student("Alice")
info_from_instance = alice.get_school_info()

Use @classmethod when you need to access class attributes or create alternative ways to construct objects.

Try it yourself

This lesson doesn't include a code challenge.

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Object Oriented Programming

2Decorators

Introduction to Decorators Property Decorator Static Method DecoratorClass Method Decorator

5Polymorphism

Method Overriding Revisited Duck Typing Abstract Classes and Methods Interface Design Recap - Shape Calculator

8Advanced OOP Concepts

Composition vs Inheritance Mixins Static and Class Methods Class Decorators Context Managers

11Design Patterns Part 2

Command Pattern Adapter Pattern Decorator Pattern Template Method Pattern State Pattern Composite Pattern

3Class Properties

Instance vs Class Variables Property Decorators Private Attributes Recap - Bank Account Manager

6Encapsulation

Public, Protected, Private Mem Access Modifiers Information Hiding Property Decorators Advanced Recap - Student Records System

9Advanced Arguments

The *args The **kwarg

12Project: Library Management

Project Overview Book and User Classes

Borrowing System

Search Functionality

Admin Interface

Testing and Integration