Constructors

What is a Constructor?

A constructor is a special function that is called when an object is created from a class.

We often use a constructor to initialize the unique properties of an object.

To create properties that are unique to each object we use instance variables.

For a dog, properties we might use instance variables for could be its name and age.

To set up these unique properties, we use something called a constructor.

Defining a Constructor

In Python, we have to define the constructor function by defining the __init__ method.

Here’s an example of how to use a constructor to create a Dog class with unique name and age properties:

class Dog:
    species = "Canis familiaris"
    number_of_legs = 4
 
    def __init__(self, name, age):
        self.name = name
        self.age = age

In this example, we created a constructor by defining the __init__ method that takes two parameters: name and age.

The self parameter is a reference to the current instance of the class and is used to access variables that belong to the class.

Now we can create multiple dog objects with different names and ages:

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

To use the constructor, we simply write the class name followed by parentheses, passing in the required arguments, just like any other function call.

In this case, we created two dog objects: dog1 with the name “Buddy” and age 3, and dog2 with the name “Max” and age 5. Each dog object has its own unique name and age instance variables, while still sharing the same species and number_of_legs class variables.

More on self

The concept of self can be a bit tricky to understand at first. So let’s break down what is happening in the constructor.

When we create a new dog object using Dog("Buddy", 3), the __init__ method is called with self referring to the new dog object being created aka its “self”.

The name of this new Dog object is set to “Buddy”, and age set to 3.