Lesson 4: Keep the returns rule out of the route

Module 10 · Lesson 4 · About 40 minutes · Needs: Lesson 1 of this module (a FastAPI route, Pydantic models)

The situation

Dev wants a "start a return" endpoint for the assistant. Aisha Rahman (cust-88) has a 4-pack of GlowLine bulbs with the wrong fitting, order 6150, delivered on 4 October. BrightCart's policy is 30 days from delivery, and the course date is 9 October 2026, so she is well inside it.

Then Priya asks for a command-line tool so her agents can start returns by hand. Two callers now need the same rule. This lesson builds the endpoint the quick way, watches the second caller drift, and rebuilds it in layers that keep one rule in one place.