The __init__ method

In Exercise 72 – creating a Pet class, you used the Pet class to create a Pet object called chubbles in the following manner:

chubbles = Pet()

Here, you’ll explore more about what happens when you create objects from a class in this manner.

Python has a special method called __init__, which is called when you initialize an object from one of our class templates. For example, building on the previous exercise, suppose you wanted to specify the height of a pet. You would add an __init__ method as follows:

class Pet:
    """
    A class to capture useful information regarding my pets, just in case
    I lose track of them.
    """
    def __init__(self, height):
        self.height = height
    
    is_human = False
    owner = 'Michael Smith'

The init method takes the height value and assigns it as an attribute of our new object. You can test this as follows:

chubbles = Pet(height=5)
chubbles.height

This will give us the following output:

out: 5

Exercise 73 – creating a Circle class

The aim of this exercise is to use the init method. You will create a new class called Circle with an init method that allows us to specify the radius and color of a new Circle object. You then use this class to create two circles:

1. Create a Circle class with a class attribute called is_shape:

   class Circle:

       is_shape = True

2. Add an init method to our class, allowing us to specify the radius and color of the specific circle:

   class Circle:

       is_shape = True

       def __init__(self, radius, color):

           self.radius = radius

           self.color = color

3. Initialize two new Circle objects with different radii and colors:

   first_circle = Circle(2, 'blue')

   second_circle = Circle(3, 'red')

Let’s have a look at some of the attributes of the Circle objects:

Figure 5.4 – Checking the attributes of our circles

In this exercise, you learned how to use the init method to set instance attributes.

Note

Any Circle objects created from our Circle class will always have is_shape = True but may have different radii and colors. This is because is_shape is a class attribute defined outside of the init method, and radius and color are instance attributes set in the init method.

Keyword arguments

As we learned in Chapter 3, Executing Python – Programs, Algorithms, and Functions, in the Basic functions section, there are two types of arguments that can go into functions – positional arguments and keyword arguments (kwargs). Remember that positional arguments are listed first and must be specified when calling a function, whereas keyword arguments are optional:

Figure 5.5 – Args and kwargs

The examples so far in this chapter have just contained positional arguments. However, you may want to provide a default value for an instance attribute. For example, you can take your previous example and add a default value for color:

class Circle:
    is_shape = True
    
    def __init__(self, radius, color='red'):
        self.radius = radius
        self.color = color

Now, if you initialize a circle without specifying a color, it will default to red:

my_circle = Circle(23)
my_circle.color

You will get the following output:

'red'

Exercise 74 – the Country class with keyword arguments

The aim of this exercise is to use keyword arguments to allow optional instance attribute inputs to be specified in the init function.

You create a class called Country, where there are three optional attributes that can be passed into the init method:

1. Create the Country class with three keyword arguments to capture details about the Country object:

   class Country:

       def __init__(self, name='Unspecified', population=None, size_kmsq=None):

           self.name = name

           self.population = population

           self.size_kmsq = size_kmsq

2. Initialize a new Country, noting that the order of parameters does not matter because you are using named arguments:

   usa = Country(name='United States of America', size_kmsq=9.8e6)

Note

Here, e is shorthand for “10 to the power of” – for instance, 2e4 == 2 x 10 ^ 4 == 20,000.

3. Use the __dict__ method to view a list of the attributes of the usa object:

   usa.__dict__

You will get the following output:

Figure 5.6 – Dictionary output of our usa object

In this exercise, you learned how keyword arguments can be used when initializing an object with a class.