Lesson Complete!

What you learned

• Why response models matter for API documentation and how they let clients know exactly what to expect
• How to create response models and attach them to your endpoints using response_model
• How to document error responses so that your API's /docs page shows both success and error shapes
• How to build a summary endpoint that aggregates data by category

Congratulations

You built a complete REST API from scratch. Your expense tracker handles creating, reading, updating, and deleting expenses, persists data to a file, and has fully documented endpoints. That is a real, working API — not a toy example.

What comes next

Your API works, but it is not production-ready yet. The next course in this path, Harden Your FastAPI, picks up where you left off — same project, new skills.

You will learn to add:

• Automated tests with FastAPI's TestClient — so you can change code without breaking things
• Middleware for logging and request timing — so you can see what hits your API
• Atomic file writes — so a crash never corrupts your data
• Environment config with Pydantic Settings — so secrets stay out of your code
• CORS headers — so a frontend can call your API
• Global error handling — so every error returns the same consistent format

Same expense tracker. No new project to learn. Just better engineering.