Stories about Software


ChessTDD 31: Look, We Caught a Bug!

A bit of time went by between when I recorded the code and when I narrated it, so pardon the unusual amount of rust there. But this episode was particularly interesting because an actual bug emerged and I fixed it. Yay for acceptance tests. After that, a second bug emerged, but I ran out of time. So there’s definitely a todo for episode 32.

What I accomplish in this clip

  • Got away from the C&P implementation of “then” for the new style of tests and implemented a usable one.
  • Discovered and fixed a bug in king’s moves.
  • Discovered another bug to fix next time.

Here are some lessons to take away:

  • When you’re stumped by behavior, particularly in integration tests, the continuous testing tool can help you run experiments very quickly.  Add a precondition assert to verify that your assumptions are correct.
  • TDD is not a catch-all against bugs, by any stretch.  I had a dumb bug in the implementation of the King class that I failed to catch, and everyone following along failed to catch (assuming someone would have reported it, anyway).  It wasn’t until I started simulating real production usage that these bugs started to be revealed.  Acceptance tests are critical.
  • The balance between ATDD and TDD is beneficial.  You’ll see going forward that when I find problems, I tend to use increasingly specific tests the way that you might be used to using step-through in the debugger.  Narrowing the scope of the problem with tests rather than the debugger has the advantage of leaving a trail of breadcrumbs that become guards against regressions as you go on.
  • This ATDD/TDD stuff works.  As you can see, I caught 2 bugs that could have escaped into production.
  • Never commit with red tests, obviously, but I also say never take a break with red tests (the way I would have to between clips).  If you have to go, comment or delete that red test, so that you can start fresh with green next time and reintroduce it.

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Amitai Schlair

Haven’t watched yet, but depending on the situation I might disagree with that last point. One way I make context-switching less expensive for myself is, once I get to green and commit, leave myself one new red test — basically fail("hey goofball, the next thing you wanted to do was in Foo::barMethod()"). Then, when I come back, I skip the procrastination step entirely, and the context-reloading step is much faster.

Erik Dietrich

For commits, a broken build seems like a high price for the team to pay for me to remember where I left off. For me personally, when taking breaks, the green invariant forces me to prefer working toward passing tests as a stopping point. I don’t need to optimize for getting back to where I left off if I get used to leaving off green.

Amitai Schlair
Shoulda been clearer. About committing, I never want to inflict known red tests on teammates. About stopping, preferably somewhere green so I can commit. About continuing, preferably with a single not-checked-in red test that reminds me what I had planned to do next, because I’m forgetful but I’ve managed to train myself to run the tests pretty often. 😉 Since it’s not checked in, once I see my past self’s reminder to my present self, I can immediately revert to my last checked-in state, see once more that it’s green, and proceed in the direction I’ve now managed to remember.… Read more »