Tuesday, 21 August 2012

The goodness of testing

Wall e-coyote testing a new rocket design
There’s a common view amongst programmers that testing (that is writing tests) is a painful thing you have to do to please your manager, or something you can avoid doing if your manager hasn’t got a clue. Let’s try to debunk that and show that testing is actually fun for the following reasons:

It guarantees code correctness. That’s quite obvious. Less obviously it forces you to think about what your code correctness actually is. By writing test, you will more easily think about those pesky edge cases that other users of your code will inevitably notice.

It gives you a safety net when you refactor. Imagine you’ve finally decided to repay some technical debt and re-write your poorly written methods (don’t tell me you haven’t got any). If you’ve got good test coverage, no worries. You can refactor and rewrite your module, increase performance, cleanup poorly written code without worrying about breaking anything. Understand anything that’s tested.

It forces you to write better APIs. By putting you in the role of a user of your code/module/API, it forces you to think about the nicest way to use your module. This is creative, fun and interesting: How, as a client of your library, would you like to be able to use it. How would you like to interact with your piece of software? What do you expect from it? What should it expects from you? You decide, and because you're hopefully lazy and imaginative, you'll come to think about something nice and neat. If anybody else uses your code in the future, you will attract a lot of developer love. Because people will go: "hum, that’s neat, and it works first time." And that's a lot of doughnuts for you.

Moses Testing the power of the LORD
You can see your test as a kind of contract about what is supposed to work now and forever more (biblical voice needed). It’s a contract between You, your future You, anyone who touches your code in the future, and also with the outside world. Why the outside world? I sometime think that everything that’s not tested doesn’t exists. It’s probably a philosophical view, but as a programmer, I like to think in binary terms. Something is there or something is not there. At least until quantum computers are on our desktops. So what about a feature that is not tested? It can break without anyone noticing, or it can break and only the users notice (very bad). So is it there or not? Well, if it can go away silently, I think it’s safer to assume it’s not there. Ask your sales team if they feel comfortable about selling stuff that can vanish at any time. Anything outside testing shouldn’t be assumed to be existing. That leads to what I deeply think about testing.

Testing does not help to make your product more reliable. Testing is your product.

Comments welcome!