Python classmethod() Method

The Python classmethod() function is a built-in function that transforms a regular method into a class method. It takes a single parameter: the function to be converted. A class method receives the class itself as its first argument (conventionally named cls) rather than an instance, which means it can access and modify class-level state. The function returns a class method object. In modern Python, the @classmethod decorator is the preferred and more readable way to define class methods. Class methods are particularly valuable for creating alternative constructors – factory methods that create instances from different data formats. A practical example is a Date.from_string("2025-01-03") method that parses a date string and returns a new Date instance. This distinguishes classmethod() from staticmethod(), which receives no implicit argument at all, and from regular instance methods which receive self.

What does classmethod() return?

The classmethod() function returns a class method object that wraps the given function, causing it to receive the class as its first argument instead of an instance.

When should you use classmethod()?

Use classmethod() (or the @classmethod decorator) when you need a method that operates on the class itself rather than an instance, especially for alternative constructors or methods that need to access class-level attributes.

The classmethod() method returns the class function for the given function.

The syntax of the classmethod() method is:

classmethod(function)

The classmethod() is considered un-pythonic in newer python versions to use the @classmethod decorator for the classmethod definition.

The syntax of the classmethod decorator is:

@classmethod
def func(cls, args....)

What is a class method?

A class method is a function that is bound to a class rather than its objects, which does not require the creation of a class instance, more like a staticmethod.

The main difference between a staticmethod and a classmethod is:

• Static method knows nothing about the class and just deals with the parameters.

• The class method works with the class since its parameters are always the class itself.

classmethod() Parameters

classmethod() method only takes a single parameter:

function - function name that needs to be converted into a class method.

Example 1: How to use classmethod()?

class Dog:
    color = 'Black'

    def printColor(cls):
        print("The color of Dog is", cls.color)

# create printColor class method

Dog.printColor = classmethod(Dog.printColor)

Dog.printColor()

The output will be as follow:

The color of Dog is Black

Here, we have a class Dog, with a member variable color assigned to black.

We also have a function printColor that takes a parameter cls and not the self we usually take.

cls method accepts the class Dog as a parameter rather than Dog’s object/instance.

Now we pass the method Dog.printColor as an argument to the function classmethod. This will convert the function to a class method to accept the first parameter as a class(i.e., Dog).

In the last line, we call printColor without creating a Dog object as we do for static methods. This prints the class variable color.

Common Use Cases

A common use case for classmethod() is defining alternative constructors, such as User.from_json(data) or Date.from_string("2025-01-03"), which parse different input formats and return new instances. Another practical scenario is creating methods that need to work with class-level configuration or state, such as a Database.get_connection() method that returns a connection using class-level settings. It is also used in inheritance hierarchies where a factory method needs to return an instance of the subclass that called it, which is possible because cls refers to the actual calling class rather than a hardcoded one, unlike staticmethod().

Rules of classmethod()

classmethod() function will return a class method for a given function.