Software's prime directive

Wednesday 7 September 2011

This morning, a coverage.py user reporting a bug concerning incorrect results said,

seeing things other than 100%, even when I know they're 100% in actuality, somehow manages to drive me up the wall

I know exactly what he means. In fact, the whole reason I started hacking on coverage.py was to keep it from reporting that docstrings weren't executed. That is, I needed to keep it from being stupid. Why wouldn't it drive you up a wall to have a tool report to you something you know isn't true?

As a crude breakdown, software does one of three things: performs some work for you, tells you something, or entertains you. It fails when it doesn't do its job, that is, when it doesn't work, tells you something wrong, or is boring. In the case of coverage.py, it's job is the second kind, it tells you things about your code. The worst thing coverage.py can do is to be wrong.

Writing software involves a huge amount of complexity. We developers need all the help we can get to master the complexity and conquer the problems we face. The last thing we need is to call on an assistant to help, and have it become another source of confusion and uncertainty.

Developer tools that give misleading advice, or warn about non-problems, or put roadblocks in your way, can be worse than no help at all. The prime directive for tools like this is Don't Be Dumber than the Developer. It's why coverage.py has pragmas for overriding its warnings. There's no way a tool is going to always get it right, especially with a dynamic language like Python. The goal is always to make it as smart as possible, but it's bound to have a simplified and limited understanding of the code, especially compared to the developer who can see the big picture. So giving the developer ways to override the savant is crucial: the developer has to remain in charge.

If you use coverage.py, and find yourself shaking your head at how stupid it has been, let me know. I want to make sure that you are in charge, and that it never causes more problems than it solves.

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