Classes in Python

Focusing first on the data, each thing or object is an instance of some class.

The primitive data structures available in Python, like numbers, strings, and lists are designed to represent simple things like the cost of something, the name of a poem, and your favorite colors, respectively.

What if you wanted to represent something much more complicated?

For example, let’s say you wanted to track a number of different animals. If you used a list, the first element could be the animal’s name while the second element could represent its age.

How would you know which element is supposed to be which? What if you had 100 different animals? Are you certain each animal has both a name and an age, and so forth? What if you wanted to add other properties to these animals? This lacks organization, and it’s the exact need for classes.

Classes are used to create new user-defined data structures that contain arbitrary information about something. In the case of an animal, we could create an Animal() class to track properties about the Animal like the name and age.

It’s important to note that a class just provides structure—it’s a blueprint for how something should be defined, but it doesn’t actually provide any real content itself. The Animal() class may specify that the name and age are necessary for defining an animal, but it will not actually state what a specific animal’s name or age is.

It may help to think of a class as an idea for how something should be defined.

Python Objects (Instances)

While the class is the blueprint, an instance is a copy of the class with actual values, literally an object belonging to a specific class. It’s not an idea anymore; it’s an actual animal, like a dog named Roger who’s eight years old.

Put another way, a class is like a form or questionnaire. It defines the needed information. After you fill out the form, your specific copy is an instance of the class; it contains actual information relevant to you.

You can fill out multiple copies to create many different instances, but without the form as a guide, you would be lost, not knowing what information is required. Thus, before you can create individual instances of an object, we must first specify what is needed by defining a class.

How To Define a Class in Python

Defining a class is simple in Python:

class Dog:
    pass

You start with the class keyword to indicate that you are creating a class, then you add the name of the class (using CamelCase notation, starting with a capital letter.)

Also, we used the Python keyword pass here. This is very often used as a place holder where code will eventually go. It allows us to run this code without throwing an error.

class Dog:

    # Initializer / Instance Attributes
    def __init__(self, name, age):
        self.name = name
        self.age = age

Instance Methods

Instance methods are defined inside a class and are used to get the
contents of an instance. They can also be used to perform operations
with the attributes of our objects. Like the __init__ method, the first argument is always self:

class Dog:

    # Class Attribute
    species = 'mammal'

    # Initializer / Instance Attributes
    def __init__(self, name, age):
        self.name = name
        self.age = age

    # instance method
    def description(self):
        return "{} is {} years old".format(self.name, self.age)

    # instance method
    def speak(self, sound):
        return "{} says {}".format(self.name, sound)

# Instantiate the Dog object
mikey = Dog("Mikey", 6)

# call our instance methods
print(mikey.description())
print(mikey.speak("Gruff Gruff"))