Summary

Recall

Often, we’ll spot really good ideas that are repeated; the repetition can form a recognizable pattern. Exploiting a pattern-based approach to software design can save the developer from wasting time trying to reinvent something already well-understood. In this chapter, we looked at a few more advanced design patterns:

• An Adapter class is a way to insert an intermediary so a client can make use of an existing class even when the class is not a perfect match. The software adapter parallels the idea of USB hardware adapters between various kinds of devices with various USB interface connectors.

• The Facade pattern is a way to create a unified interface over a number of objects. The idea parallels the facade of a building that unifies separate floors, rooms, and halls into a single space.

• We can leverage the Flyweight pattern to implement a kind of lazy initialization. Instead of copying objects, we can design Flyweight classes that share a common pool of data, minimizing or avoiding initialization entirely.

• When we have closely related classes of objects, the Abstract Factory pattern can be used to build a class that can emit instances that will work together.

• The Composition pattern is widely used for complex document types. It covers programming languages, natural languages, and markup languages, including XML and HTML. Even something like the filesystem with a hierarchy of directories and files fits this design pattern.

• When we have a number of similar, complex classes, it seems appropriate to create a class following the Template pattern. We can leave gaps or openings in the template into which we can inject any unique features.

These patterns can help a designer focus on accepted, good design practices. Each problem is, of course, unique, so the patterns must be adapted. It’s often better to make an adaptation to a known pattern and avoid trying to invent something completely new.

Synopsis

In this chapter, we went into detail on several more design patterns, covering their canonical descriptions as well as alternatives for implementing them in Python, which is often more flexible and versatile than traditional object-oriented languages. The Adapter pattern is useful for matching interfaces, while the Facade pattern is suited to simplifying them. Flyweight is a complicated pattern and only useful if memory optimization is required. Abstract Factories allow the runtime separation of implementations depending on configuration or system information. The Composite pattern is used universally for tree-like structures. A Template method can be helpful for breaking complex operations into steps to avoid repeating the common features.