Python classmethod()

Python’s built-in function classmethod() prefixes a method definition in a class as an annotation @classmethod. This annotation transforms a normal instance method into a class method. The difference between class and instance method is that Python passes the class itself as a first implicit argument of the method rather than the instance on which it is called.

In this tutorial, I’ll show you one of Python’s little-known secrets that separate the intermediates from the experts: class methods. You may know the difference between an instance method and a class method conceptually. (If you don’t, this tutorial is for you.) But do you also know how to create a class method in Python? If not, read on, because this tutorial will show you!

Syntax Class Method

Syntax: 
classmethod(function)    # <--- This is considered unpythonic
@classmethod                        # <--- As a prefix before the used method

The most Pythonic way to declare a class method is the following decorator syntax:

class C:
    @classmethod
    def f(cls, arg1, arg2, ...):
        None

How to Call a Class Method?

There are two ways of using a class method:

  • You can call it on a class such as C.f(arg1, arg2, ...), or
  • You can call it on an instance such as C().f(arg1, arg2, ...).

Note that Python implicitly passes the class itself—in Python, everything including a class is an object—as a first argument. Roughly speaking, if you call C.f(arg1, arg2, ...), Python really runs f(C, arg1, arg2, ...) with the class object passed as a first argument.

Example Class Method

When to use a class method? You’ll find that class methods are often used in a factory pattern. A factory pattern is one of the most popular software development patterns: a so-called factory method creates new instances of a certain type. It is responsible for actually creating the instances from a given class definition.

In the following example, you define a class Coffee that represents a real coffee serving in a coffee bar. There are many specialities such as Cappuccino (80% milk, 20% coffee), Espresso Macchiato (30% milk, 70% coffee), and Latte Macchiato (95% milk, 5% coffee).

You use the class methods to allow users to conveniently call factory methods instead of setting the milk level themselves. Instead of calling Coffee(80, 20, 'Arrabica'), a user can just call cappuccino() to create the same instance of the class Coffee. This simplifies the creation of new instances and can improve the readability and usability of your code:

class Coffee:

  def __init__(self, milk, beans):
    self.milk = milk # percentage
    self.coffee = 100-milk # percentage
    self.beans = beans


  def __repr__(self):
    return f'Milk={self.milk}% Coffee={self.coffee}% Beans={self.beans}'


  @classmethod
  def cappuccino(cls):
    return clf(80, 'Arrabica')
  

  @classmethod
  def espresso_macchiato(cls):
    return cls(30, 'Robusta')
  

  @classmethod
  def latte(cls):
    return cls(95, 'Arrabica')


print(Coffee.cappuccino())
print(Coffee.espresso_macchiato())
print(Coffee.latte())

The three lines of output are:

Milk=80% Coffee=20% Beans=Arrabica
Milk=30% Coffee=70% Beans=Robusta
Milk=95% Coffee=5% Beans=Arrabica

You can run this example in our interactive Jupyter Notebook:

Interactive Example Class Method

Exercise: Can you create another coffee specialty?

Class Method Naming Convention: cls vs self

In the previous example, you’ve seen the naming convention of the first argument of a class method: it’s the three characters cls—short for “class”, but we cannot take this because class is a reserved keyword. You can use any name you want but using the name cls is the expected convention and it’s highly recommended. The naming convention of the first implicit argument of instance methods is the name self.

Related Article: Self in Python

Class Method is a Function Decorator

Decorators help to add functionality to existing code without having to modify the code itself. Decorators are so-called because they decorate code, they do not modify the code, but they make the code do different things using decoration. Now that we have understood closures, we can work our way step by step to understanding and using decorators.

The @classmethod is a function decorator. It’s short for calling classmethod(m) for the method m that you would decorate.

Here’s the same simplified coffee example without using a decorator and by using classmethod() instead:

class Coffee:

  def __init__(self, milk, beans):
    self.milk = milk # percentage
    self.coffee = 100-milk # percentage
    self.beans = beans
    


  def __repr__(self):
    return f'Milk={self.milk}% Coffee={self.coffee}% Beans={self.beans}'


  def cappuccino(cls):
    return cls(80, 'Arrabica')
  

Coffee.cappuccino = classmethod(Coffee.cappuccino)
print(Coffee.cappuccino())

The output is the same:

Milk=80% Coffee=20% Beans=Arrabica

However, this is not the recommended way—use a decorator with the @ annotation instead!

Related Article: Decorators

Class Method vs Instance Method

If you don’t use the @classmethod annotator, you obtain an instance method per default. The instance method requires that the first argument self is a reference to the instance itself on which the method is called. The class method expects a reference to the class, not the instance, as a first argument cls. Thus, the difference between class and instance methods is that Python passes the class rather than the instance (object) as a first implicit argument.

Here’s a minimal example of a class and an instance method:

class C:

    @classmethod
    def f(cls):
        None


    # instance method
    def g(self):
        None



# call class method:
C.f()

# call instance method:
C().g()

There are two ways of using a class method:

  • You can call it on a class such as C.f(arg1, arg2, ...), or
  • You can call it on an instance such as C().f(arg1, arg2, ...).

But there’s only one way of using an instance method:

  • You can call it on an instance such as C().g().

Class Method vs Static Method

You may know static methods from programming languages such as C++ or Java. They’re methods that exist independently of whether or not you created an instance of the class. That’s why they don’t use any instance variable in the method body. If you want to use a static method in Python, you need to use the @staticmethod annotation rather than the @classmethod annotation. The difference is that static methods don’t expect a reference to either the instance or the class as an implied first argument.

Here’s an example comparing class methods, instance methods, and static methods:

class C:

    @classmethod
    def f(cls):
        None


    # instance method
    def g(self):
        None


    @staticmethod
    def h():
        None


# call class method:
C.f()

# call instance method:
C().g()


# call static method:
C.h()

Summary

Python’s built-in function classmethod() prefixes a method definition in a class as an annotation @classmethod.

class C:


    @classmethod
    def f(cls, arg1, arg2, ...):
        None

This annotation transforms a normal instance method into a class method.

The difference between class and instance method is that Python passes the class itself as a first implicit argument of the method rather than the instance on which it is called:

class C:


    @classmethod
    def f(cls, arg1, arg2, ...):
        None


    # instance method
    def g(self, arg1, arg2, ...):
        None

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Where to Go From Here?

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