Class attributes and methods

Class attributes are shared by all objects of a class. Think of them as common features that apply to every object created from the class.

Imagine a class for creating cats. All cats share some common traits:

• All cats are mammals.

Instead of giving each cat object the same "mammal" information, we can store it at the class level. This makes it easy to access and saves space.

How do you define class attributes?

Class attributes are defined outside any methods, directly under the class definition. For example:

class Cat:
    # Class attribute
    species = "Mammal"

• species is a class attribute because it belongs to the Cat class and is the same for all Cat objects.

Accessing class attributes

You can access class attributes:

• Using the class name:

  print(Cat.species)  # Output: Mammal
  

• Using an object:

  my_cat = Cat()
  print(my_cat.species)  # Output: Mammal
  

All objects share the same class attribute. Changing the class attribute affects all objects:

Cat.species = "Feline"
print(my_cat.species)  # Output: Feline

What are class methods?

Class methods are actions related to the class itself, not just individual objects. These methods:

• Use the @classmethod decorator.
• Take cls (short for class) as the first parameter instead of self.

Why use class methods?

Class methods are useful when you want to:

• Modify or interact with class-level data (like class attributes).
• Create objects in a specific way (we’ll see an example).

Defining class methods

Here’s how to define a class method:

class Cat:
    species = "Mammal"  # Class attribute

    @classmethod
    def set_species(cls, new_species):
        cls.species = new_species  # Modify the class attribute

• The @classmethod decorator tells Python it’s a class method.
• cls refers to the class itself, similar to how self refers to the object.

Using class methods

You call class methods using either the class name or an object:

# Call using the class name
Cat.set_species("Feline")

# Call using an object
my_cat = Cat()
my_cat.set_species("Big Cat")

print(Cat.species)  # Output: Big Cat

When you change the class attribute through a class method, it affects all objects.

Class methods to create objects

Class methods can also be used to provide alternate ways to create objects. For example:

class Dog:
    species = "Mammal"

    def __init__(self, name, age):
        self.name = name
        self.age = age

    @classmethod
    def puppy(cls, name):
        # Create a "puppy" object with a default age of 1
        return cls(name, 1)

# Create objects using the class method
puppy = Dog.puppy("Buddy")
print(puppy.name)  # Output: Buddy
print(puppy.age)   # Output: 1

• The puppy class method creates a Dog object with a default age of 1.

Key differences: instance vs. class

Feature	Instance	Class
Attributes	Unique to each object	Shared by all objects
Methods	Use self to access data	Use cls to modify class data
When to Use?	For object-specific data	For shared data or alternate constructors

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Example: class attributes and methods together

class Car:
    # Class attribute
    wheels = 4

    def __init__(self, brand, color):
        self.brand = brand  # Instance attribute
        self.color = color  # Instance attribute

    @classmethod
    def change_wheels(cls, num):
        cls.wheels = num  # Modify class attribute

# Access class attribute
print(Car.wheels)  # Output: 4

# Create objects
car1 = Car("Toyota", "red")
car2 = Car("Tesla", "blue")

# Change class attribute using class method
Car.change_wheels(6)

print(car1.wheels)  # Output: 6
print(car2.wheels)  # Output: 6
print(Car.wheels)   # Output: 6