Write automated tests is one of the best ways to ensure high quality code and a peace of mind for both developer and her managers. In this session we'll learn how to write tests using pytest. We will start by introducing pytest and we will see how to use it to write unittest for brand new code. TDD style. Then we'll take a look at the case where you already have a lot of code without tests and consider how can we approach that problem. We'll see how to write integration tests and how to mock the rest of the world.