Implementing a reusable package

Tip

This section includes a lot of source code. Remember that you don't have to type it all in by hand; a complete copy of the quantities package is provided as part of the sample code that can be downloaded for this chapter.

Start by creating the directory named quantities to hold our new package. Inside this directory, create a new file named quantity.py. This module will hold our implementation of a quantity—that is, a value together with its associated units.

While you don't need to understand object-oriented programming techniques to work through this book, this is the one place where we need to use object-oriented programming. This is because we want the user to be able to print a quantity directly, and the only way to do that in Python is to use objects. Don't worry, though—this code is very straightforward, and we'll take it one step at a time.

In the quantity.py module, enter the following Python code:

class Quantity(object):
    def __init__(self, value...