At LocalStack, we are building a platform that enables development and testing of cloud applications on your local machine. The core is an [open source AWS emulator] that is primarily written in Python. It is among the top Python projects on GitHub, and has seen a massive uptake in contributions over the past two years. Many Python software developers and architects will relate to the struggles of maintaining a large and complex Python codebase, while keeping developer teams productive. In this talk, we'll explore how we at LocalStack tackle these as we re-create AWS for local development. We'll explain our approaches to automating around AWS specifications, building a highly modular and pluggable system to make it easy for teams to integrate their components, the software patterns we use to keep devs productive, as well as our approach to automated contract testing using pytest.