The template pattern

The template pattern is useful for removing duplicate code; it's intended to support the Don't Repeat Yourself principle we discussed in Chapter19, When to Use Object-Oriented Programming. It is designed for situations where we have several different tasks to accomplish that have some, but not all, steps in common. The common steps are implemented in a base class, and the distinct steps are overridden in subclasses to provide custom behavior. In some ways, it's like a generalized strategy pattern, except similar sections of the algorithms are shared using a base class. Here it is in the UML format:

A template example

Let's create a car sales reporter as an example. We can store records of sales in an SQLite database table. SQLite is a simple file-based database engine that allows us to store records using SQL syntax. Python includes SQLite in its standard library, so there are no extra modules required.

We have two common tasks we need to perform:

• Select all sales of new...