With interpreted languages it is trivial, you just try parts of your code. For python there's for example ipython which makes the experience really great. Also

With interpreted languages it is trivial, you just try parts of your code. For python there's for example ipython which makes the experience really great. Also I don't agree with GP, there are places that require unit tests, especially if multiple people are working on a project. You could be a developer that never made a bug in your life, but others might not be as great :) besides many bugs also happen by changing other parts of the code. Some of your assumptions might have been 100% correct when you wrote the code, but someone can change different part of the code and now your function is called with values that were previously impossible. The great thing about statically typed languages is that they might detect large part of these bugs, but you're out of luck with dynamically typed languages. Example of this was the Python 2 -> Python 3 migration. Having a large test suite helped tremendously.