More often than not, the software we write directly interacts with what we would label as “dirty” services. In layman’s terms: services that are crucial to our application, but whose interactions have intended but undesired side-effects—that is, undesired in the context of an autonomous test run.
For example: perhaps we’re writing a social app and want to test out our new ‘Post to Facebook feature’, but don’t want to actually post to Facebook every time we run our test suite.
unittest library includes a subpackage named
unittest.mock—or if you declare it as a dependency, simply
mock—which provides extremely powerful and useful means by which to mock and stub out these undesired side-effects.