I recently shared how a team I worked on designed an API using code. The goal was to show that code can be used as a tool for design, and how prototyping can give us fast feedback on the quality of our design. In this writeup we’ll look at how we might use code to design API behavior. We’ll use Python and FastAPI like we did in the last writeup, so you may want to read it first before moving forward.

I was on a team that needed to design a new API. As a Design First team, we’d normally start off with an OpenAPI file. But this time we started with some code. We opened up our IDE, created a Python file, and started coding models and endpoints. It might seem like we switched to a Code First approach since we started with code. But this wasn’t the case for us.