I've really been working hard to make sure that every piece of code that I commit to listparser is backed by unit tests to ensure that the code does what I expect it to do. But a while back I had the idea that it would be great to have some kind of program that could watch the unit tests run and produce a webpage showing me exactly what lines of code did and did not get tested.

Today the thought occurred to me again, and within minutes I found coverage.py, which does exactly what I had envisioned. It ran my unit tests, produced a highlighted webpage, and showed me that I hadn't tested eight lines of code. Minutes later I was down to five lines of untested code.

Is a metric like "97% code coverage" a good indicator of the strength of the unit tests? No. It's trivially easy to produce code that doesn't account for things:

if everything_is_okay:
    print "sweet"

(In this example, what if not everything_is_okay?!) So, code coverage isn't a guarantee that all your bases are...um, covered. It is, however, another useful tool.