classmethod() in Python

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.

You can also use @classmethod decorator for classmethod definition.

Syntax: 

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

Where, 

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

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. 

Class method vs Static Method

• A class method takes cls as the first parameter while a static method needs no specific parameters.
• A class method can access or modify the class state while a static method can’t access or modify it.
• In general, static methods know nothing about the class state. They are utility-type methods that take some parameters and work upon those parameters. On the other hand class methods must have class as a parameter.
• We use @classmethod decorator in python to create a class method and we use @staticmethod decorator to create a static method in python.

Example of classmethod in Python

Example 1: Create a simple classmethod

In this example, we are going to see a how to create classmethod, for this we created a class with geeks name with member variable course and created a function purchase which prints the object.

Now we passed the method geeks.purchase into classmethod which converts the methods to a class method and then we call the class function purchase without creating a function object.

Python3

class geeks:
    course = 'DSA'

    def purchase(obj):
        print("Puchase course : ", obj.course)


geeks.purchase = classmethod(geeks.purchase)
geeks.purchase()

Output:

Puchase course :  DSA

Example 2: Create class method using classmethod()

Python3

# 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 3: Factory method using Class method

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

Python3

# 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)

    def display(self):
        print("Name : ", self.name, "Age : ", self.age)

person = Person('mayank', 21)
person.display()

Output:

Name :  mayank Age :  21

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, ...):
       ....

Where,

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

Note:

• 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

# 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
25
True