Object-Oriented Programming (OOP) in Python 3

Classes vs Instances

Classes allow you to create user-defined data structures. Classes define functions called methods, which identify the behaviors and actions that an object created from the class can perform with its data.

In this tutorial, you’ll create a Dog class that stores some information about the characteristics and behaviors that an individual dog can have.

A class is a blueprint for how to define something. It doesn’t actually contain any data. The Dog class specifies that a name and an age are necessary for defining a dog, but it doesn’t contain the name or age of any specific dog.

While the class is the blueprint, an instance is an object that’s built from a class and contains real data. An instance of the Dog class is not a blueprint anymore. It’s an actual dog with a name, like Miles, who’s four years old.

Put another way, a class is like a form or questionnaire. An instance is like a form that you’ve filled out with information. Just like many people can fill out the same form with their own unique information, you can create many instances from a single class.

Class Definition

You start all class definitions with the class keyword, then add the name of the class and a colon. Python will consider any code that you indent below the class definition as part of the class’s body.

Here’s an example of a Dog class:

# dog.py

class Dog:
    pass

The body of the Dog class consists of a single statement: the pass keyword. Python programmers often use pass as a placeholder indicating where code will eventually go. It allows you to run this code without Python throwing an error.

The Dog class isn’t very interesting right now, so you’ll spruce it up a bit by defining some properties that all Dog objects should have. There are several properties that you can choose from, including name, age, coat color, and breed. To keep the example small in scope, you’ll just use name and age.

You define the properties that all Dog objects must have in a method called .__init__(). Every time you create a new Dog object, .__init__() sets the initial state of the object by assigning the values of the object’s properties. That is, .__init__() initializes each new instance of the class.

You can give .__init__() any number of parameters, but the first parameter will always be a variable called self. When you create a new class instance, then Python automatically passes the instance to the self parameter in .__init__() so that Python can define the new attributes on the object.

Update the Dog class with an .__init__() method that creates .name and .age attributes:

# dog.py

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

Make sure that you indent the .__init__() method’s signature by four spaces, and the body of the method by eight spaces. This indentation is vitally important. It tells Python that the .__init__() method belongs to the Dog class.

In the body of .__init__(), there are two statements using the self variable:

1. self.name = name creates an attribute called name and assigns the value of the name parameter to it.
2. self.age = age creates an attribute called age and assigns the value of the age parameter to it.

Attributes created in .__init__() are called instance attributes. An instance attribute’s value is specific to a particular instance of the class. All Dog objects have a name and an age, but the values for the name and age attributes will vary depending on the Dog instance.

On the other hand, class attributes are attributes that have the same value for all class instances. You can define a class attribute by assigning a value to a variable name outside of .__init__().

For example, the following Dog class has a class attribute called species with the value "Canis familiaris":

# dog.py

class Dog:
    species = "Canis familiaris"

    def __init__(self,