This book is my attempt to share with the world the journey I’ve taken from “hacking” to “software engineering”. It’s mainly about testing, but there’s a lot more to it, as you’ll soon see.
I want to thank you for reading it.
If you bought a copy, then I’m very grateful. If you’re reading the free online version, then I’m still grateful that you’ve decided it’s worth spending some of your time on. Who knows, perhaps once you get to the end, you’ll decide it’s good enough to buy a real copy for yourself or for a friend.
If you have any comments, questions, or suggestions, I’d love to hear from you. You can reach me directly via firstname.lastname@example.org, or on Twitter @hjwp. You can also check out the website and my blog, and there’s a mailing list.
I hope you’ll enjoy reading this book as much as I enjoyed writing it.
“Who are you, why are you writing this book, and why should I read it?” I hear you ask.
I’m still quite early on in my programming career. They say that in any discipline, you go from apprentice, to journeyman, and eventually, sometimes, on to master. I’d say that I’m—at best—a journeyman programmer. But I was lucky enough, early on in my career, to fall in with a bunch of TDD fanatics, and it made such a big impact on my programming that I’m burning to share it with everyone. You might say I have the enthusiasm of a recent convert, and the learning experience is still a recent memory for me, so I hope I can still empathise with beginners.
When I first learned Python (from Mark Pilgrim’s excellent Dive Into Python), I came across the concept of TDD, and thought “Yes. I can definitely see the sense in that”. Perhaps you had a similar reaction when you first heard about TDD? It sounds like a really sensible approach, a really good habit to get into—like regularly flossing your teeth or something.
Then came my first big project, and you can guess what happened—there was a client, there were deadlines, there was lots to do, and any good intentions about TDD went straight out of the window.
And, actually, it was fine. I was fine.
At first I knew I didn’t really need TDD because it was a small website, and I could easily test whether things worked by just manually checking it out. Click this link here, choose that drop-down item there, and this should happen. Easy. This whole writing tests thing sounded like it would have taken ages, and besides, I fancied myself, from the full height of my three weeks of adult coding experience, as being a pretty good programmer. I could handle it. Easy.
Then came the fearful goddess Complexity. She soon showed me the limits of my experience.
The project grew. Parts of the system started to depend on other parts. I did
my best to follow good principles like DRY (Don’t Repeat Yourself), but that
just led to some pretty dangerous territory. Soon I was playing with multiple
inheritance. Class hierarchies 8 levels deep.
I became scared of making changes to my code. I was no longer sure what depended on what, and what might happen if I changed this code over here, oh gosh, I think that bit over there inherits from it—no, it doesn’t, it’s overriden. Oh, but it depends on that class variable. Right, well, as long as I override the override it should be fine. I’ll just check—but checking was getting much harder. There were lots of sections to the site now, and clicking through them all manually was starting to get impractical. Better to leave well enough alone, forget refactoring, just make do.
Soon I had a hideous, ugly mess of code. New development became painful.
Not too long after this, I was lucky enough to get a job with a company called Resolver Systems (now PythonAnywhere), where Extreme Programming (XP) was the norm. They introduced me to rigorous TDD.
Although my previous experience had certainly opened my mind to the possible benefits of automated testing, I still dragged my feet at every stage. “I mean, testing in general might be a good idea, but really? All these tests? Some of them seem like a total waste of time … What? Functional tests as well as unit tests? Come on, that’s overdoing it! And this TDD test/minimal-code-change/test cycle? This is just silly! We don’t need all these baby steps! Come on, we can see what the right answer is, why don’t we just skip to the end?”
Believe me, I second-guessed every rule, I suggested every shortcut, I demanded justifications for every seemingly pointless aspect of TDD, and I came out seeing the wisdom of it all. I’ve lost count of the number of times I’ve thought “Thanks, tests”, as a functional test uncovers a regression we would never have predicted, or a unit test saves me from making a really silly logic error. Psychologically, it’s made development a much less stressful process. It produces code that’s a pleasure to work with.
So, let me tell you all about it!
My main aim is to impart a methodology—a way of doing web development, which I think makes for better web apps and happier developers. There’s not much point in a book that just covers material you could find by googling, so this book isn’t a guide to Python syntax, or a tutorial on web development per se. Instead, I hope to teach you how to use TDD to get more reliably to our shared, holy goal: clean code that works.
With that said: I will constantly refer to a real practical example, by building a web app from scratch using tools like Django, Selenium, jQuery, and Mock. I’m not assuming any prior knowledge of any of these, so you should come out of the other end of this book with a decent introduction to those tools, as well as the discipline of TDD.
In Extreme Programming we always pair-program, so I’ve imagined writing this book as if I was pairing with my previous self, having to explain how the tools work and answer questions about why we code in this particular way. So, if I ever take a bit of a patronising tone, it’s because I’m not all that smart, and I have to be very patient with myself. And if I ever sound defensive, it’s because I’m the kind of annoying person that systematically disagrees with whatever anyone else says, so sometimes it takes a lot of justifying to convince myself of anything.
I’ve split this book into three parts.
On to a little housekeeping…
The following typographical conventions are used in this book:
Constant width bold
Occasionally I will use the symbol:
to signify that some of the content has been skipped, to shorten long bits of output, or to skip down to a relevant bit.
This element signifies a tip or suggestion.
This element signifies a general note or aside.
This element indicates a warning or caution.
Code examples are available at https://github.com/hjwp/book-example/; you’ll find branches for each chapter there (eg, https://github.com/hjwp/book-example/tree/chapter_03). You’ll also find some suggestions on ways of working with this repository at the end of each chapter.
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