Unit Tests Are Good For You

Some 5-6 years ago, I became a mother, biologically implausible as it seems. At the time, I was CTO at Jadestone and that’s when I started to think that programmers writing their own unit tests is a paradigm shift of the same magnitude as object-orientation was in the 90s. So I started trying to convert the developers (and others) to the unit testing mentality, with varying success – some became as convinced as myself, others were very reluctant to write any tests at all, and most seemed to end up in some in-between state: “I guess it’s a good thing to do, and I will if there’s time, but…” It seems like a lot of developers end up somewhere in that middle state, with writing unit tests being something a bit analogous to cleaning your room every week. You know it’s a good thing to do, and you do it because Mum (yep, that’s me) is telling you you have to, but you don’t really want to.

Even at Shopzilla, where the builds fail unless a certain percentage of the code is covered by unit tests (this is a great practice despite the shortfalls of code coverage as a measurement of test quality), there are issues with people’s motivation to write unit tests. Far too much of our code is being ‘exercised’ by the unit tests with no checking of expected results, and/or without sufficient branch coverage.

I remember creating slides at Jadestone with what I still think are really good reasons to write lots of unit tests: it improves the code quality by helping you find bugs, making your code testable forces you to design it in a modular way, having great regression tests gives you the courage to do the aggressive refactoring that is required for long-term productivity, and so on. But after a few years of working with it, I’ve come to the conclusion that the main reason really is – hold your ears, I can feel years of nagging coming out all at once:


People thinking “I will write unit tests if I have the time” or “when I’m done with the code” are just plain wrong. True, just writing the code needed for a feature or bug fix takes less time than also writing unit tests. But checking that the fix/feature works if it is a part of a slightly complex system? Starting or restarting one of the Shopzilla websites takes something like 3-7 minutes. Once you’re up, you can typically test almost anything by navigating to a single URL, but if you didn’t get the fix right, you have to re-fix, re-build and re-start. If the fix is in a shared library, you need to re-build and re-install the library, then re-build and re-start the site. This takes ages. Usually a lot more than the time it takes to verify the fix with a unit test.

And with multiplayer games, for instance, the situation is even harder. To verify a Shopzilla-style website bug, all you need is normally a single URL to reproduce the problem. With more complex clients, you may have to bring a couple of clients and a server into a given error state in order to reproduce/verify a bug. This can be a nightmare. Even with unit tests, you’ll obviously have to do this, but if you do TDD, you’ll have written a test that reproduces the observed error before trying to fix it. Seeing that the unit test used to be broken but isn’t any longer is great for your confidence and usually means you only need to do the work to reproduce the bug “in reality” once. I’m not sure it is necessary to write tests before you write all your code, but I think it is a good idea to try to do so. And you definitely don’t want to write all your unit tests after you’ve written all the feature code.

I don’t try to convert people into unit tests as much as I used to, but if you do use unit tests properly, not only will you write code that is of higher quality, more modular and easier to refactor to make it support future features and changes. You’ll be adding working features quicker, too. Really, kids, you should clean your rooms every week.



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