Me too, of course. But it isn't always easy to test a function in isolation. If it takes complex data structures as input, then the only reasonable way to provide that input may be calling it from another piece of the code.

Yes. Therefore the best thing is to codesign program and tests in parallel. Make your code testable from the beginning. That will lead to a more flexible design also. (In a perfect world I would follow this advice myself... ;-)) Cheers, Markus