The Factory Pattern

Design patterns are reusable programming solutions that have been used in various real-world contexts, and have proved to produce expected results. They are shared among programmers and continue being improved over time. This topic is popular thanks to the book by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, titled Design Patterns: Elements of Reusable Object-Oriented Software.

Gang of Four: The book by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides is also called the Gang of Four book for short (or GOF book for even shorter).

Here is a quote about design patterns from the Gang of Four book:

A design pattern systematically names, motivates, and explains a general design that addresses a recurring design problem in object-oriented systems. It describes the problem, the solution, when to apply the solution, and its consequences. It also gives implementation hints and examples. The solution is a general arrangement of objects and classes that solve the problem. The solution is customized and implemented to solve the problem in a particular context.

There are several categories of design patterns used in object-oriented programming, depending on the type of problem they address and/or the types of solutions they help us build. In their book, the Gang of Four present 23 design patterns, split into three categories: creational, structural, and behavioral.

Creational design patterns are the first category we will cover throughout this chapter, and Chapters 2, The Builder Pattern and Chapter 3, Other Creational Patterns. These patterns deal with different aspects of object creation. Their goal is to provide better alternatives for situations where direct object creation, which in Python happens within the __init__() function, is not convenient.

See https://docs.python.org/3/tutorial/classes.html for a quick overview of object classes and the special __init__() method Python uses to initialize a new class instance.

We will start with the first creational design pattern from the Gang of Four book: the factory design pattern. In the factory design pattern, a client (meaning client code) asks for an object without knowing where the object is coming from (that is, which class is used to generate it). The idea behind a factory is to simplify the object creation process. It is easier to track which objects are created if this is done through a central function, compared to letting a client create objects using a direct class instantiation. A factory reduces the complexity of maintaining an application by decoupling the code that creates an object from the code that uses it.

Factories typically come in two forms—the factory method, which is a method (or simply a function for a Python developer) that returns a different object per input parameter, and the abstract factory, which is a group of factory methods used to create a family of related objects.

In this chapter, we will discuss:

• The factory method
• The abstract factory