What is the use of the "classmethod" decorator in Python?

Question

Jayant Kumar · Accepted Answer

Table of Contants
Introduction
The classmethod decorator in Python is used to define methods that operate on the class itself rather than on instances of the class. This means that classmethod methods receive the class as the first argument (cls) instead of an instance (self). This allows these methods to access and modify class-level data, making them useful for operations that are related to the class as a whole rather than to individual instances.

How the classmethod Decorator Works
A method defined with the classmethod decorator receives the class itself as the first argument, allowing it to access and modify class-level attributes and methods. This is different from instance methods, which receive the instance as the first argument.
Syntax:
class ClassName:
    @classmethod
    def method_name(cls, arguments):
        # Method body
        pass

**@classmethod**: Decorator used to define a class method.
**method_name**: The name of the class method.
**cls**: The class itself, passed as the first argument.
**arguments**: Parameters passed to the class method.

Example with a Class Method:
class Account:
    interest_rate = 0.05

@classmethod
    def set_interest_rate(cls, rate):
        cls.interest_rate = rate

@classmethod
    def get_interest_rate(cls):
        return cls.interest_rate

# Using class methods without creating an instance
Account.set_interest_rate(0.07)
print(Account.get_interest_rate())  # 0.07

In this example, set_interest_rate and get_interest_rate are class methods that modify and access the class-level attribute interest_rate.

Key Uses of the classmethod Decorator

Modifying Class State: Use classmethod to modify class-level attributes or manage class-level state that is shared across all instances of the class.
Factory Methods: Create factory methods that return instances of the class using different initialization parameters or from different data sources.
Accessing Class Data: Access class-level data without needing to create an instance of the class, which can be useful for utility functions or configuration methods.

Example with Factory Methods:
class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

@classmethod
    def from_birth_year(cls, name, birth_year):
        from datetime import datetime
        current_year = datetime.now().year
        age = current_year - birth_year
        return cls(name, age)

person = Person.from_birth_year("Alice", 1990)
print(person.name)  # Alice
print(person.age)   # 34 (assuming the current year is 2024)

In this example, from_birth_year is a class method that serves as a factory method to create a Person instance based on the birth year.

Conclusion
The classmethod decorator in Python is a powerful tool for defining methods that operate on the class itself rather than on instances. By using classmethod, you can modify class-level attributes, create factory methods, and access class data without needing to instantiate the class. This decorator enhances class design by allowing you to manage class-level operations and maintain a clean, organized codebase.

What is the use of the "classmethod" decorator in Python?

Table of Contants

Introduction

How the `classmethod` Decorator Works

Syntax:

Example with a Class Method:

Key Uses of the `classmethod` Decorator

Example with Factory Methods:

Conclusion

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