Mocks are objects created as substitutes for real objects with the purpose of testing part of the behavior of the real objects. If you have seen the movie Body Snatchers, you might already have an understanding of the basic idea. Generally speaking, mocking is useful only when the real objects under test are expensive to create, such as a database connection, or when testing could have undesirable side effects; for instance, we might not want to write to the file system or a database.
In this recipe, we will test a nuclear reactor—not a real one, of course! This nuclear reactor class performs a factorial calculation that could, in theory, cause a chain reaction with a nuclear disaster as consequence. We will mock the factorial computation with a mock, using the mock package.