I'm gradually bringing an old project into good test coverage. Coveralls is pretty strict -- 66% and it's still showing a red badge!
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I find putting a coveralls.io badge on my projects is a great incentive to review my tests and what functionality I'm exercising.
RPython is lovely. I get a fast interpreter, but I can still use the full Python toolset for unit testing (e.g. coverage with coveralls.io).
Coveralls (and other coverage sites e.g. Jenkins) are great. Knowing which code is untested helps find bugs and unused code.