Testing parallel, non-deterministic code
Hands-on practice on testing multiple processes. Also, a refresher on Calculus :)
Deterministic tests - introduction to monkeypatching
Some parts of your code are hard to test - API calls, randomized or long-running calculations cannot be tested effectively. Luckily, there are a bunch of tools to help us bypass such limitations. This article describes one of them - monkeypatching.