The post I wrote recently on recycling tests in TDD got quite a few responses. I’m going to take this opportunity to respond to some of the points that got raised.
Do we really need to use the term “recycling”?
The TDD cycle as popularly taught includes the instruction to “write a failing test”. The point of my article was to observe that there are two ways to do that:
write a new test that fails
change an existing, passing test to make it fail
It’s this second approach that I’m calling “recycling”. Alistair Cockburn says that “it’s a mystery this should need a name” and it probably doesn’t. However, I’ve regularly seen novice TDD-ers get into a mess when making the current test pass causes other test(s) to fail. Their safety net is compromised and they have a few options, none of which seem very appealing:
Roll back to last green
Comment out the failing test(s)
Modify the failing test(s) to make them pass again
Whichever way you go to get out, you’ll want to try to avoid painting yourself into a similar corner in future.
Why do tests that used to pass start failing?
Ron Jeffries suggests that this will only happen if the tests don’t “say something universally true about the problem and solution.” Several people (including George Dinwiddie and Sandro Mancuso) demonstrated that this problem can be solved by writing a series of tests that each say something “universally true.” However, to me, this seems like a similar approach to that recommended by Alistair Cockburn in his “Thinking Before Programming” post.
I’m a big fan of thinking before programming. In the courses that I deliver, I routinely prevent students from touching the keyboard until they’ve thought their way around the problem. But, it’s just not realistic to expect that […]