Another important aspect of functional programming relevant to writing testable Python code is that functions should not have side effects. In other words, functions take inputs and convert them to outputs; they don’t do anything else. This approach is aspirational rather than entirely realistic—after all, saving to the database is a side effect, and most applications would be fairly boring if they offered absolutely no way to modify the data. It just happens to be the case that our outlier detection engine can be close to side effect-free because we do not create files, save to a database, or push results to some third-party service. With most applications, however, we do not tend to be so lucky.
Click through for an excerpt from the draft of an upcoming book as well as a bit of elucidation on key points. The specific language I’m talking about here is Python but the concepts apply to most languages.