Having a framework to automatically run all of your unit tests is important, but in a strongly dynamic language like Python, it can be difficult to find all of the tests to run in a batch.
[nose] makes the whole process much easier, by automatically finding all modules containing unittest.TestCase subclasses, allowing nonsubclassed simple test functions, and generally providing a strong framework for easing your testing needs. Plus, it’s less magical than [py.test].
[nose]: http://www.somethingaboutorange.com/mrl/projects/nose (nose: a discovery-based unittest extension)
[py.test]: http://codespeak.net/py/dist/test.html (a framework to perform python testing)