Class Method in Python [ English ]

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1. Introduction

A class method in Python is a method that is bound to the class rather than the object (instance). It can access and modify class-level data (class variables) but not instance-specific data directly.

Class methods are defined using the @classmethod decorator.

2. Definition

A class method is a method that:

• Takes cls (class reference) as its first parameter
• Works with class variables
• Is shared among all objects of the class

3. Syntax

class ClassName:
    @classmethod
    def method_name(cls):
        # code

4. Basic Example

class Student:
    school = "ABC School"

    @classmethod
    def show_school(cls):
        print("School:", cls.school)

Student.show_school()

Output

School: ABC School

5. Using Class Method with Objects

class Student:
    school = "ABC School"

    @classmethod
    def show_school(cls):
        print(cls.school)

s1 = Student()
s1.show_school()

6. Key Difference: self vs cls

7. Modifying Class Variables

class Student:
    school = "ABC School"

    @classmethod
    def change_school(cls, new_name):
        cls.school = new_name

Student.change_school("XYZ School")

print(Student.school)

Output

XYZ School

8. Class Method as Alternative Constructor

Class methods are often used to create alternative constructors.

class Student:
    def __init__(self, name):
        self.name = name

    @classmethod
    def from_string(cls, data):
        name = data.split("-")[0]
        return cls(name)

s1 = Student.from_string("Rahul-20")

print(s1.name)

9. When to Use Class Methods

Use class methods when:

• You need to work with class variables
• You want to modify class-level data
• You need alternative constructors
• You want functionality related to the class, not instances

10. Important Points

• Defined using @classmethod decorator
• First parameter is always cls
• Can be called using class name or object
• Cannot directly access instance variables

11. Summary

• A class method is bound to the class, not the object.
• It uses cls to refer to the class.
• It is used to access or modify class variables.
• It can act as an alternative constructor.
• Defined using the @classmethod decorator.