Part IV. Sustainable Test-Driven Development

This part discusses the qualities we look for in test code that keep the development “habitable.” We want to make sure the tests pull their weight by making them expressive, so that we can tell what’s important when we read them and when they fail, and by making sure they don’t become a maintenance drag themselves. We need to apply as much care and attention to the tests as we do to the production code, although the coding styles may differ. Difficulty in testing might imply that we need to change our test code, but often it’s a hint that our design ideas are wrong and that we ought to change the production code.

We’ve written up these guidelines as separate chapters, but that has more to do with our need for a linear structure that will fit into a book. In practice, these qualities are all related to and support each other. Test-driven development combines testing, specification, and design into one holistic activity.¹

1. For us, a sign of this interrelatedness was the difficulty we had in breaking up the material into coherent chapters.