17 Classes and Attributes

Learning Objectives

Understand what a class is and the syntax
Understand class and instance attributes

17.1 Classes and Object Instantiation

A class is a blueprint for creating objects. Once a class is defined, you can instantiate objects (create instances of that class) like this:

class Dog:
    pass  # placeholder for now

# Instantiate a Dog object
my_dog = Dog()
another_dog = Dog()

my_dog and another_dog are two separate objects created from the Dog class.

17.2 Instance Attributes (Object-Specific)

Instance attributes belong to a specific object. They are usually defined in the init method using self:

class Dog:
    def __init__(self, name, age):
        self.name = name   # instance attribute
        self.age = age     # instance attribute


dog1 = Dog("Buddy", 3)
dog2 = Dog("Max", 5)

print(dog1.name, dog1.age)  # Buddy 3
print(dog2.name, dog2.age)  # Max 5

In the same way as you pass variables to a function, when you instantiate a class to create an object of the class you can pass in variables. You can also set defaults in the same way.

Exercise 1 - printing class attributes

Level:
dir(MyClass) outputs a list of all the attributes. This includes methods, inherited attributes, and special methods - which we will look at next.

Try print(dir(MyClass)) to see the attributes present.

You can also use print(MyClass.__dict__) or vars(MyClass) to look at all those only defined on the class object.

17.3 Modifying Instance Attributes

You can access and modify instance attributes by using the . directly on the object:

dog1.age += 1
dog1.name = "Buddy Jr."
print(dog1.name, dog1.age)  # Buddy Jr. 4

Each object maintains its own copy of instance attributes, so modifying one object does not affect others.

17.4 Class Attributes (Shared Across Objects)

Class attributes belong to the class itself and are shared by all objects:

class Dog:
    species = "Canine"  # class attribute shared by all instances

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

dog1 = Dog("Buddy")
dog2 = Dog("Max")

print(dog1.species)  # Canine
print(dog2.species)  # Canine

17.5 Modifying Class Attributes

You can modify a class attribute either for the whole class or for a specific instance:

Change for all objects

Dog.species = "Dog"
print(dog1.species)  # Dog
print(dog2.species)  # Dog

Override for one instance

dog1.species = "Super Dog"
print(dog1.species)  # Super Dog
print(dog2.species)  # Dog

Note: Assigning to dog1.species creates a new instance attribute on that object, leaving the class attribute unchanged for other instances.

17.6 Global and Local Variables

Global variables are defined outside functions or classes and can be accessed anywhere:

count = 0  # global variable

Local variables exist only inside a function or method:

def example():
    x = 10  # local variable
    print(x)

example()

Remember: Instance variables (self.name) are specific to an object Class variables (Dog.species) are shared across all instances

Variable Type	Scope
Global	Entire program
Local	Inside a function/method
Instance	Specific object
Class	Shared by all objects of the same class

17.7 Summary

You can straight away see the benefits of OOP in having many objects of a class with different attributes