Python classmethod

Unlike Static Methods, classmethod in Python bound to a Class. So, we don’t have to create an instance or object of a class to call this class method. A Python classmethod receives cls (class) as an implicit first argument, just like a standard method receives self as the first argument. This cls allows you to access the class variables, methods, and the static methods of the class.

You can define any method as the Python classmethod by using the @classmethod decorator or classmethod() function. Either way, it works, but it is always advisable to go for the first option. The syntax of the Python classmethod is

Class A:
    @classmethod
    def function_name(cls, arg1, arg2,....):
         ........
         ......

Or
Class A:
    def function_name(cls, arg1, arg2,....):
        ........
         ......
 
A.function_name = classmethod(function_name)

You can call the class method using ClassName.MethodName() or ClassName().MethodName(). Both ways will return the classmethod result.

In this section, we show how to create or define a classmethod in Python programming language using @classmethod and classmethod() with examples.

Python classmethod using Decorator

In this example, we are creating a class method called message using the Python @classmethod decorator. Within this method, cls.__name__ returns the class name (Employee), and cls.company returns the class variable company value (Tutorial Gateway).

# Python Class Method
 
class Employee:
 
    company = 'Tutorial Gateway'
 
    @classmethod
    def message(cls):
        print('The Message is From %s Class' %cls.__name__)
        print('The Company Name is %s' %cls.company)
 
Employee.message()
 
print('-----------')
Employee().message() # Other way of calling classmethod

classmethod using the decorator output

The Message is From Employee Class
The Company Name is Tutorial Gateway
-----------
The Message is From Employee Class
The Company Name is Tutorial Gateway

Python classmethod using classmethod() function

Here, we are using the Python classmethod() function to create a class method. From the below Python code, the Employee.printValue = classmethod(Employee.printValue) statement convert the method to the class method.

class Employee:
 
    value = 100
 
    def printValue(cls):
        print('The Value = %d' %cls.value)
 
Employee.printValue = classmethod(Employee.printValue)
Employee.printValue()
The Value = 100

Call Static Method from classmethod in Python

In this example, we show how to call Python Static Methods within the class method. Here, we created a static method called func_msg(), which prints a welcome message. Next, we defined the message class method that returns a class variable company and the class name. Within the same function, we are calling the static method using the cls.methodname.

class Employee:

    company = 'Tutorial Gateway'
 
    @classmethod
    def message(cls):
        print('The Company Name is %s' %cls.company)
        print('The Message is From %s Class' %cls.__name__)
        cls.func_msg()
 
    @staticmethod
    def func_msg():
        print("Welcome to Python Programming")
  
Employee.message()
Python Classmethod Example 3

Here, Instead of printing the message, we are finding the sum and average. First, we created a Static Method that accepts three arguments and returns some of those three. Next, we defined a Python classmethod that calls the static method using the cls. Within the class method, it finds returns the average of static method result. 

class Employee:
 
    company = 'Tutorial Gateway'
 
    @staticmethod
    def add(a, b, c):
        return a + b + c

    @classmethod
    def avg(cls):
        x = cls.add(10, 20, 40)
        return (x / 3)
  
average = Employee.avg()
print('The Average Of three Numbers = ', average)

classmethod calling static method output

The Average Of three Numbers =  23.333333333333332

Alter class variable using clasmethod in Python

In this example, we are going to create a Python classmethod that accepts an argument and assigns the value to the class variable. It means, when you call this method, it replaces the company text with the new text that you provide as an argument value. It helps to hide the class variables and allows the end-users to work with the class variable.

class Employee:
 
    company = 'Tutorial Gateway'
 
    @classmethod
    def func_newName(cls, new_Name):
        cls.company = new_Name
  
emp = Employee()

print(Employee.company)
print(emp.company)

print('----------')
Employee.func_newName('Python')
 
print(Employee.company)
print(emp.company)

Output of altering the class variable using a classmethod

Tutorial Gateway
Tutorial Gateway
----------
Python
Python

Real-time Examples of classmethod in Python

For example, if the client receives the Employee information in a long string, and details separated by the – (or any other delimiter). Instead of performing splitting operations from his end, we can create a Python class method and allow them to use it.

In this Python classmethod example, we initialized fullname, age, gender, and salary. Next, we created a classmethod that split the given string based on – and returns those values. I suggest you refer to the Python split string article to understand the split function

class Employee:

    def __init__(self, fullname, age, gender, salary):
        self.fullname = fullname
        self.age = age
        self.gender = gender
        self.salary = salary

    @classmethod
    def func_string_split(cls, employee_str):
        fullname, age, gender, salary = employee_str.split('-')
        return cls(fullname, age, gender, salary)
  
emp_from_csv1 = 'Suresh-27-Male-120000'
emp_from_csv2 = 'John-29-Male-100000'
emp_from_csv3 = 'Tracy-25-Female-155000'
 
emp1 = Employee.func_string_split(emp_from_csv1)
print(emp1.fullname)
print(emp1.gender)
print(emp1.salary)
print(emp1.age)
 
print('----------')
emp3 = Employee.func_string_split(emp_from_csv3)
print(emp3.fullname)
print(emp3.gender)
print(emp3.age)
Suresh
Male
120000
27
----------
Tracy
Female
25

It is another example of classmethod. Here, we are splitting the date string to Day, Month, and Year. Here, we used the Python map function for this splitting.

class Date:
 
    def __init__(self, day = 0, month = 0, year = 0):
        self.day = day
        self.month = month
        self.year = year
 
    @classmethod
    def string_to_Date(cls, string_Date):
        day, month, year = map(int, string_Date.split('-'))
        return cls(day, month, year)
 
dt = Date.string_to_Date('31-12-2018')
print(dt.day)
print(dt.month)
print(dt.year)

classmethod and map function output

31
12
2018