unittest has an important and highly useful capability that
doctest lacks. You can tell
unittest how to create a standardized environment for your unit tests to run inside, and how to clean up that environment when it's done. This ability to create and later destroy a standardized test environment is a test fixture. While test fixtures don't actually make any tests possible that were impossible before, they can certainly make them shorter and less repetitive.
Many programs need to access a database for their operation, which means that many of the units these programs are made of also access a database. The point is that the purpose of a database is to store information and make it accessible ...