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).
miniblog.
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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.
I'm gradually bringing an old project into good test coverage. Coveralls is pretty strict -- 66% and it's still showing a red badge!
Coveralls (and other coverage sites e.g. Jenkins) are great. Knowing which code is untested helps find bugs and unused code.