Much of the industry discussion around software testing over the last couple of decades has been focused on web services, but there are lots of different types of software systems that have different testing needs. This talk will first explore the differences and similarities between testing web services and testing safety- and mission-critical software systems, such as those used on spacecraft. We will then consider a rubric for thinking about the verification needs of different types of software based on attributes of the software and the environments in which it runs. Finally, we will examine a real-world example of using pytest to test Varda Space Industries' spacecraft software, showcasing many of pytest's power features, such as its fixtures and extensive hook system, as well as Python language features such as generators, context managers, and threading, that enable easy-to-use tools for testing against real-time telemetry streams and generating rich test report output.