Manual testing is essential to ensuring that our software works as expected and does not contain critical bugs. However, manual testing is severely limited because bugs may be introduced every time a change is made in the code. We can't possibly expect to manually test our entire program at every commit.
Nowadays, automated testing is a standard practice in software engineering. In this recipe, we will briefly cover important aspects of automated testing: unit tests, test-driven development, test coverage, and continuous integration. Following these practices is absolutely necessary in order to produce high-quality software.
Python has a native unit-testing module that you can readily use (unittest). Other third-party unit testing packages exist, such as py.test or nose, which we have chosen here. nose makes it a bit easier to write a test suite, and it has a library of external plugins. Your users don't need that extra dependency unless they want...