My first encounter with Test-Driven Development (TDD) was back in 2002. At that time, it wasn't as mainstream as it is today, and I remember watching two developers writing some tests first and then implementing the functionality later. I thought it to be quite a strange way to write a code, and I promptly forgot about it. It was not until 2004, when I was involved with a challenging project, that I remembered TDD again. We were faced with a messy code that was difficult to test and every change seemed to create a series of new bugs. I thought, why not give TDD a shot and see how it worked? Suffice to say, TDD changed my outlook on software development. We stopped writing messy spaghetti code, and started writing better designed, more maintainable code. Regression failures dropped drastically. I was hooked.

Perhaps, like me, you face some challenges in a project and want to see how TDD can help you. Or, maybe you've heard a lot of people in the industry sing the praises of TDD and you're wondering what all the fuss is about. Maybe you've been reading about how TDD will be an essential skill in the near future, and want to get up to speed on it. No matter what your motivation, I hope this book will help you reach your goal.

TDD is a lot more than just a library or an API; it is a different way of developing software. In this book, we'll discuss how to apply this process to writing Python software. We're in luck, because Python has fantastic support for TDD right out of the box. In fact, unit testing has been an integral part of the Python standard library from the Python 2.1 release back in April 2001. Numerous improvements have been added since then, and the latest version that ships with Python 3.4 has a ton of exciting features that we'll explore over the course of this book.