Classes Class vs. Instance Members

Learning objective: By the end of this lesson, students will be able to compare and contrast class and instance members in Python.

Class vs. instance members

In Python, attributes and methods (members) are categorized into two types based on whether they belong to instances of the class or the class itself:

Instance Attributes and Methods: These are linked to individual instances of a class. Each object created from the class has its own copy of instance attributes. For example, in a Dog class, each dog object might have its own name and age attributes.
Class Attributes and Methods: These belong to the class as a whole, not to any individual instance. All instances of the class share the same class attributes. This means that if one instance changes a class attribute, the change is reflected across all other instances. These are intended to be accessed on the class only, not an instance (although accessing them on the instance is technically possible).

To demonstrate class attributes, let’s add a next_id class attribute to the Dog class that can be used to assign an id to each dog instance:

class Dog():
    next_id = 1

    def __init__(self, name, age=0):
        self.name = name
        self.age = age
        # assign the current value of `next_id` to this instance
        self.id = Dog.next_id
        # increment the class attribute `next_id` so the next dog gets a new ID
        Dog.next_id += 1

    def bark(self):
        print(f'{self.name} says woof!')

    def __str__(self):
        return f'Dog #{self.id} named {self.name} is {self.age} years old.'

Note how the Dog.next_id class attribute is being accessed within the __init__ method.

💡 Note: Technically, instances can also access class members via self because Python will look for a member on the class if it’s not found on the instance.

Let’s check that it worked:

harry = Dog('Harry', 2)

print(harry)

maggie = Dog('Maggie')

print(maggie)

Cool, now let’s see how class methods are created by adding a get_total_dogs method.

Add this to the bottom of the Dog class:

    def __str__(self):
        return f'Dog #{self.id} named {self.name} is {self.age} years old.'

# new code below

    @classmethod
    def get_total_dogs(cls):
        # cls represents the Dog class
        return cls.next_id - 1

There are only two differences when defining a class method:

The @classmethod decorator.
The naming convention of the first parameter is to use cls instead of self.

💡 Decorators in programming are used to implement metaprogramming (when a program has knowledge or manipulates itself). In Python, decorators are used to modify the behavior of a function or class.

Let’s test out the new class method:

spot = Dog('Spot', 2)
diogee = Dog('Diogee')

# class methods are called on the class, not an instance
print(Dog.get_total_dogs()) 
# prints: an integer representing however many dogs you've created!