Customizing constructors in Python

We want to initialize instances of the Rectangle class with the values of both width and height. After we create an instance of a class, Python automatically calls the __init__ method. Thus, we can use this method to receive both the width and height arguments. We can then use these arguments to initialize attributes with the same names. We can think of the __init__ method as the equivalent of a constructor in other object-oriented programming languages.

The following lines create a Rectangle class and declare an __init__ method within the body of the class:

class Rectangle:
    def __init__(self, width, height):
        print("I'm initializing a new Rectangle instance.")
        self.width = width
        self.height = height

This method receives three arguments: self, width, and height. The first argument is a reference to the instance that called the method. We used the name self for this argument. It is important to notice that self is not a Python keyword. It is just the name for the first argument, and it is usually called self. The code within the method prints a message indicating that the code is initializing a new Rectangle instance. This way, we will understand when the code within the __init__ method is executed.

Then, the following two lines create the width and height attributes for the instance and assign them the values received as arguments with the same names. We use self.width and self.height to create the attributes for the instance. We create two attributes for the Rectangle instance right after its creation.

The following lines create two instances of the Rectangle class named rectangle1 and rectangle2. The Python console will display I'm initializing a new Rectangle instance. after we enter each line in the Python console:

rectangle1 = Rectangle(293, 117)
rectangle2 = Rectangle(293, 137)

The preceding screenshot shows the Python console. Each line that creates an instance specifies the class name followed by the desired values for both the width and the height as arguments enclosed in parentheses. If we take a look at the declaration of the __init__ method within the Rectangle class, we will notice that we just need to specify the second and third arguments (width and height). Also, we just need to skip the required first parameter (self). Python resolves many things under the hood. We just need to make sure that we specify the values for the required arguments after self to successfully create and initialize an instance of Rectangle.

After we execute the previous lines, we can check the values for rectangle1.width, rectangle1.height, rectangle2.width, and rectangle2.height.

The following line will generate a TypeError error and won't create an instance of Rectangle because we missed the two required arguments: width and height. The specific error message is TypeError: __init__() missing 2 required positional arguments: 'width' and 'height'. The error message is shown in the following screenshot:

rectangleError = Rectangle()