Introduction

In the past decade the Python community has seen a wealth of riches spring up in the area of web development. Frameworks and tools have made it easier than ever to use Python for web applications, with some focused on particular domains, others on particular footprints, and still others on particular deployment strategies. Most of these frameworks have built upon WSGI, the Web Server Gateway Interface, which became part of the Python standard library in version TK. WSGI provides some conventions for applications and servers to communicate with one another, much as it’s spiritual predecessor, CGI, provided conventions for executing scripts via a web server.

With the inclusion of WSGI, it’s possible to begin developing a web application by simply picking and choosing pieces that seem best for the task at hand. Indeed, some projects do just that. So why use a larger framework like Django, Pylons, or Blue Bream? Frameworks build upon WSGI to provide a reasonable set of defaults, a set of conventions, for getting started with development and focusing on the specific problem at hand. It’s possible to spend time evaluating libraries that map a URL to a view, but a framework’s developers have already (presumably) done such an evaluation, and chosen one that they feel will work well with the other parts of the framework. TK:Community

A framework is general purpose by definition, but that doesn’t mean your use of it must be. Put another way, most frameworks support a variety of databases, platforms, and deployment infrastructures. But just because you use that framework doesn’t mean you need to, as well. A good framework will help you get up to speed more quickly, and will let you target things for your environment when needed.

Django is a popular, powerful web framework for Python. It has lots of “batteries” included, and makes it easy to get up and going. But all of the power means you can write low quality code that still seems to work. Effective Django development means building applications that are testable, maintainable, and scalable – not only in terms of traffic or load, but in terms of being able to add developers to projects. When we’re talking about Effective Django, we’re really talking about software engineering for web applications. The examples and the details we’re going to talk about are Django specific, but the ideas and principles are not.

So what does Effective Django mean? It means using Django in a way that emphasizes writing code that’s cohesive, testable, and scalable. What do each of those words mean? Well “cohesive” code is code that is focused on doing one thing, and one thing alone. It means that when you write a function or a method, that it does one thing and does it well. This is directly related to writing testable code: code that’s doing too much is often difficult to write tests for. When I find myself thinking, “Well, this piece of code is just too complex to write a test for, it’s not really worth all the effort,” that’s a signal that I need to step back and focus on simplifying it. Testable code is code that makes it straight-forward to write tests for, and that’s easy to diagnose problems with. Finally, we want to write scalable code. That doesn’t just mean it scales in terms of performance, but that it also scales in terms of your team and your team’s understanding. Applications that are well tested are easier for others to understand (and easier for them to modify), which means you’re more able to improve your application by adding engineers.

Part of being able to effectively use Django is understanding what’s available to you, and what the restrictions are. Frameworks are necessarily general purpose tools, which is great: the abstractions and tools they provide allow us to begin working immediately, without delving into the details. At some point, however, it’s useful to understand what the framework is doing for you. Whether you’re trying to stretch in a way the framework didn’t imagine, or you’re just trying to diagnose a mysterious bug, you have to look inside the black box and gain a deeper understanding. After reading Effective Django you should have an understanding of how Django’s pieces fit together, how to use them to engineer web applications, and where to look to dig deeper.

My goal is to convince you of the importance of these principles, and provide examples of how to follow them to build more robust Django applications. I’m going to walk through building a contact management application iteratively, building tests as I go.