`classmethod()`

The built-in classmethod() function is a decorator that transforms a method into a class method. A class method receives the class itself as its first argument, enabling it to access or modify class-level data rather than instance data. This functionality is particularly useful for creating multiple constructors, which allow for different ways to instantiate the class:

Pythonmargherita.py
      
    
class Pizza:
    def __init__(self, ingredients):
        self.ingredients = ingredients

    @classmethod
    def margherita(cls):
        return cls(["mozzarella", "tomatoes"])

    def __str__(self):
        return f"Pizza with {' and '.join(self.ingredients)}"

# Usage
print(Pizza.margherita())  # Output: Pizza with mozzarella and tomatoes

classmethod() Syntax

Python Syntax
      
    
@classmethod
def method(cls, *args, **kwargs):
    ...

Arguments

Argument	Description
`method`	A method to be converted into a class method

Return Value

Returns a class method object that can be called on both the class and its instances.

classmethod() Examples

As an alternative constructor:

Pythonpoint.py
      
    
class ThreeDPoint:
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z

    @classmethod
    def from_sequence(cls, sequence):
        return cls(*sequence)

    def __repr__(self):
        return f"ThreeDPoint(x={self.x}, y={self.y}, z={self.z})"

# Usage
print(
    ThreeDPoint.from_sequence((4, 8, 16))
)  # Output: ThreeDPoint(x=4, y=8, z=16)

classmethod() Common Use Cases

The most common use cases for the @classmethod include:

Creating multiple constructors for flexibility in instantiation
Implementing factory methods to encapsulate complex object creation logic
Modifying class-level attributes that apply to all instances

classmethod() Real-World Example

A common real-world application of @classmethod is to create factory methods that return pre-configured instances of a class. For example, a Pizza class might have class methods for creating specific types of pizzas:

Pythonpizzas.py
      
    
class Pizza:
    def __init__(self, ingredients):
        self.ingredients = ingredients

    @classmethod
    def margherita(cls):
        return cls(["mozzarella", "tomatoes"])

    @classmethod
    def prosciutto(cls):
        return cls(["mozzarella", "tomatoes", "ham"])

    def __str__(self):
        return f"Pizza with {' and '.join(self.ingredients)}"

# Usage
print(
    Pizza.margherita()
)  # Output: Pizza with mozzarella and tomatoes
print(
    Pizza.prosciutto()
)  # Output: Pizza with mozzarella and tomatoes and ham