classmethod() in Python - GeeksforGeeks

The classmethod() is an inbuilt function in Python, which returns a class method for a given function.

Syntax: classmethod(function)

Parameter :This function accepts the function name as a parameter.

Return Type:This function returns the converted class method.

classmethod() methods are bound to a class rather than an object. Class methods can be called by both class and object. These methods can be called with a class or with an object. The examples below clearly illustrate this.

Example #1:

# Python program to understand the classmethod 
  
class Student: 
      
    # create a variable 
    name = "Geeksforgeeks"
      
    # create a function 
    def print_name(obj): 
        print("The name is : ", obj.name) 
          
# create print_name classmethod 
# before creating this line print_name() 
# It can be called only with object not with class 
Student.print_name = classmethod(Student.print_name) 
  
# now this method can be called as classmethod 
# print_name() method is called a class method 
Student.print_name() 

Output:

The name is :  Geeksforgeeks

Example #2:

# Python program to understand the classmethod 
  
class Subject: 
      
    # create a variable 
    favorite_subject = "Networking"
      
    # create a function 
    def favorite_subject_name(obj): 
        print("My favorite_subject_name is : ", 
                           obj.favorite_subject) 
          
# create favorite_subject_name classmethod 
# before creating this line favorite_subject_name() 
# It can be called only with object not with class 
Subject.favorite_subject_name = classmethod(Subject.favorite_subject_name) 
  
# now this method can be called as classmethod 
# favorite_subject_name() method is called as class method 
Subject.favorite_subject_name() 

Output:

My favorite_subject_name is :  Networking

Uses of classmethod() classmethod() function is used in factory design patterns where we want to call many functions with the class name rather than object.

The @classmethod Decorator:

The @classmethod decorator, is a built-in function decorator which is an expression that gets evaluated after your function is defined. The result of that evaluation shadows your function definition.
A class method receives the class as the implicit first argument, just like an instance method receives the instance.

Syntax:

class C(object):
    @classmethod
    def fun(cls, arg1, arg2, ...):
       ....

fun: the function that needs to be converted into a class method
returns: a class method for function.

A class method is a method which is bound to the class and not the object of the class.
They have the access to the state of the class as it takes a class parameter that points to the class and not the object instance.
It can modify a class state that would apply across all the instances of the class. For example, it can modify a class variable that would be applicable to all the instances.

In the below example we use a staticmethod() and classmethod() to check if a person is an adult or not.

# Python program to demonstrate  
# use of a class method and static method. 
from datetime import date 
  
class Person: 
    def __init__(self, name, age): 
        self.name = name 
        self.age = age 
      
    # a class method to create a  
    # Person object by birth year. 
    @classmethod
    def fromBirthYear(cls, name, year): 
        return cls(name, date.today().year - year) 
      
    # a static method to check if a 
    # Person is adult or not. 
    @staticmethod
    def isAdult(age): 
        return age > 18
  
person1 = Person('mayank', 21) 
person2 = Person.fromBirthYear('mayank', 1996) 
  
print (person1.age) 
print (person2.age) 
  
# print the result 
print (Person.isAdult(22)) 

Output:

21
22
True

My Personal Notes

Recommended Posts: