Class Methods

In Python, class methods are methods that work on the class itself, not just its instances.

They are defined using the @classmethod decorator, and the first parameter is always cls (a reference to the class).

Example 1: Basic Class Method

class Example:
    @classmethod
    def describe_class(cls):
        return f"This is the {cls.__name__} class."
 
print(Example.describe_class()) # Output: This is the Example class.

Here,

• The @classmethod decorator tells Python that describe_class is a class method.
• The first argument cls refers to the class, not an object.
• You can call it directly on the class (Example.describe_class()), without creating an instance.

Why Use Python Class Methods?

Class methods are useful in these cases:

1. Factory Methods: To create objects with special logic.
2. Alternative Constructors: To allow multiple ways to create objects.
3. Class-level Utilities: For operations that relate to the class, not just instances.

Example 2: Factory Method

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age
 
    @classmethod
    def from_birth_year(cls, name, birth_year):
        age = 2025 - birth_year
        return cls(name, age)
 
# Create object using normal constructor
p1 = Person("Alice", 30)
 
# Create object using class method (alternative constructor)
p2 = Person.from_birth_year("Bob", 2000)
 
print(p1.name, p1.age)  # Output: Alice 30
print(p2.name, p2.age)  # Output: Bob 25

Here,

• from_birth_year is a class method that calculates age from birth year.
• Instead of writing Person("Bob", 25), you can call Person.from_birth_year("Bob", 2000).
• This makes the code cleaner and allows multiple ways to create objects.

👉 Next tutorial: Python Class Methods as Alternative Constructors