This tutorial introduces to writing tests with Python. We briefly discuss benefits of test-driven development and start hands-on working with py.test, a popular tool for writing and running automated tests in a simple manner. Step by step, we walk through the writing of tests, using debugging facilities, the setup of application specific test state and the organisation of tests in a larger project. Eventually we look at how to write project specific testing extensions and we discuss existing ones for generating HTML pages, running javascript tests or distributing tests across multiple machines.