Hillel Wayne - Beyond Unit Tests: Taking Your Testing to the Next Level - PyCon 2018

Most of the non-joyful experience of writing tests lies in the context preparation, not so much in the expected outputs. I think we need a modern version of design by contract instead. Its one of the most powerful techniques of testing software is unfortunately often ignored and overlooked these days... Some less known things about design by contract: - you can easily test way more than types in preconditions and postconditions - extreme web controller postcondition example: every time there is a 200 response to a POST /new request, there should be at least one document more in storage than there was before the request. (caveat: unless there are deletes - then sum the deletes up too) - things important to the business look a ton like contracts - e.g. a diff of two legal documents always contains the full content (list of words) of both documents (never accidentally swallow content!) - your postconditions can surprisingly often fully describe the desired function behavior - with sufficient amount of pre and postconditions, your property tests look like "exercise the module or entire system with random actions" -> unit and integration tests (see Hilel Wayne's excellent video on this topic [this video] ) Some things that could really help modernize design by contract - Test at a certain sampling rate in production for expensive contracts (do not turn them off) - Report contract violations via observability mechanisms (tracing, sentry etc) - Better language support for contract DSLs