On Test-Driven Development

I was having a conversation with someone the other day about unit testing. OK, actually I was interviewing someone for a Quality Engineering position on my team. We were talking about the difference between white-box tests that quality engineers write and tests that developers write.

I suggested that good white-box testers test the functionality and the failure cases (the intent of the function) and developers test the code that they’ve written (the function as coded). This then lead me to a new revelation around test-first development methodologies (or possibly reminded me of something I had forgotten).

I have been a proponent of writing tests first, since I first started doing Extreme Programming and read Kent Beck’s original book, Extreme Programming Explained: Embrace Change while working at Bootleg Networks (thanks Carmine for making me do that, by the way). Although admittedly, like many developers, I haven’t always been that rigorous at following that rule.

What I like about writing the tests before the function is that it clarifies my thinking about what the function should do, it alerts me to the corner cases, it gives me reasons to consider if the function is doing too much, and it gives me a way to instantly know if the function works once it is written. Writing the tests first also makes sure that the tests are written at all. Once the function is coded, it sometimes gets tempting to move on to the next bit of coding work with the intention of filling in the tests later.

What I hadn’t considered about writing the tests before the code is that it puts me into a quality mindset without having any bias to the code as I’d written it. I’m divorced from my own blind-spots around my coding. This actually leads me to writing better tests because I have no assumptions about how the code should work or fail. I’m testing the functionality, not the code.

Maybe I’d thought about this before, but I hadn’t really considered that benefit recently until that moment. Now, when I start to get lazy about writing my unit tests before my implementation, I’ll have a better reason to keep up my discipline.

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