What is a Pattern?

One of the earliest efforts to study and document design patterns was a book titled Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides, who later became known as the Gang of Four (GoF). This book is so influential that many consider the 23 design patterns in the book as fundamental to software engineering itself.

In reality, the patterns were written primarily for object-oriented programming languages, and it had code examples in C++ and Smalltalk. As we will see shortly, many of these patterns might not be even required in other programming languages with better higher-order abstractions such as Python.

The 23 patterns have been broadly classified by their type as follows:

While a detailed explanation of each pattern would be beyond the scope of this book, it would be interesting to identify some of these patterns in Django itself:

While these patterns are mostly of interest to those studying the internals of Django, the pattern under which Django itself can be classified under—is a common question.

Model-View-Controller (MVC) is an architectural pattern invented by Xerox PARC in the 70s. Being the framework used to build user interfaces in Smalltalk, it gets an early mention in the GoF book.

Today, MVC is a very popular pattern in web application frameworks. Beginners often ask the question—is Django an MVC framework?

The answer is both yes and no. The MVC pattern advocates the decoupling of the presentation layer from the application logic. For instance, while designing an online game website API, you might present a game's high scores table as an HTML, XML, or comma-separated (CSV) file. However, its underlying model class would be designed independent of how the data would be finally presented.

MVC is very rigid about what models, views, and controllers do. However, Django takes a much more practical view to web applications. Due to the nature of the HTTP protocol, each request for a web page is independent of any other request. Django's framework is designed like a pipeline to process each request and prepare a response.

Django calls this the Model-Template-View (MTV) architecture. There is separation of concerns between the database interfacing classes (Model), request-processing classes (View), and a templating language for the final presentation (Template).

If you compare this with the classic MVC—"Model" is comparable to Django's Models, "View" is usually Django's Templates, and "Controller" is the framework itself that processes an incoming HTTP request and routes it to the correct view function.

If this has not confused you enough, Django prefers to name the callback function to handle each URL a "view" function. This is, unfortunately, not related to the MVC pattern's idea of a View.

In 2002, Martin Fowler wrote Patterns of Enterprise Application Architecture, which described 40 or so patterns he often encountered while building enterprise applications.

Unlike the GoF book, which described design patterns, Fowler's book was about architectural patterns. Hence, they describe patterns at a much higher level of abstraction and are largely programming language agnostic.

Fowler's patterns are organized as follows:

Almost all of these patterns would be useful to know while architecting a Django application. In fact, Fowler's website at http://martinfowler.com/eaaCatalog/ has an excellent catalog of these patterns. I highly recommend that you check them out.

Django also implements a number of these patterns. The following table lists a few of them:

Yes, of course. Patterns are discovered all the time. Like living beings, some mutate and form new patterns: take, for instance, MVC variants such as Model–view–presenter (MVP), Hierarchical model–view–controller (HMVC), or Model View ViewModel (MVVM).

Patterns also evolve with time as better solutions to known problems are identified. For example, Singleton pattern was once considered to be a design pattern but now is considered to be an Anti-pattern due to the shared state it introduces, similar to using global variables. An Anti-pattern can be defined as commonly reinvented but a bad solution to a problem.

Some of the other well-known books which catalog patterns are Pattern-Oriented Software Architecture (known as POSA) by Buschmann, Meunier, Rohnert, Sommerlad, and Sta; Enterprise Integration Patterns by Hohpe and Woolf; and The Design of Sites: Patterns, Principles, and Processes for Crafting a Customer-Centered Web Experience by Duyne, Landay, and Hong.