FastAPI From Zero
FastAPI is the framework that made building Python APIs feel modern. You write a function with type hints, and FastAPI gives you — for free — request parsing, data validation, JSON serialization, and a complete interactive API documentation page. That "for free" isn't marketing: it falls directly out of one clever idea, which is that your Python type hints become the single source of truth for how the API behaves. Learn that idea and FastAPI stops being a bag of decorators and becomes predictable.
This guide builds the mental model first the whole way: why type hints drive everything, what Pydantic is
doing under each model, how the Depends() system turns dependency injection into plain functions, and
when async actually helps (and when it quietly hurts). By the end you'll build a real, validated,
authenticated, tested API and understand every layer.
📝 This teaches the framework. It assumes you know Python — functions, classes, and especially type hints (Python From Zero covers them; FastAPI leans on them hard). Helpful background: REST APIs Explained and What a Framework Even Is.
How to read this
Read in order — it builds one API (a small book service) and adds a layer per phase. Many pure-Python snippets here are runnable right on the page (Pydantic models, validation); FastAPI app code that needs a running server is shown with the commands to run it yourself. Phases carry difficulty badges.
The phases
Part 1 — The core (🟢 Basic)
- What FastAPI Is & Your First App 🟢 — ASGI, your first endpoint,
uvicorn, and the automatic interactive docs. - Path Operations & Parameters 🟢 — routes, path and query parameters, and how type hints parse and validate them.
- Pydantic Models & Validation 🟢 — request bodies as models, and validation that comes free from your types.
Part 2 — A real application (🟡 Intermediate)
4. Response Models & Status Codes 🟡 — shaping output, hiding internal fields, and honest HTTP statuses.
5. Dependency Injection with Depends() 🟡 — reusable dependencies for auth, DB sessions, and shared logic.
6. Async & Concurrency 🟡 — async def vs def, when async helps, and the trap that blocks the event loop.
7. Databases with SQLModel 🟡 — persistence, sessions via dependencies, and CRUD.
Part 3 — Production (🔴 Advanced → 🟢)
8. Authentication & Security 🔴 — OAuth2 password flow, JWT, and security as dependencies.
9. Testing & Project Structure 🟡 — TestClient, pytest, and structuring a real project with routers.
10. Production & Where to Go Next 🟢 — deployment, background tasks, async pitfalls, and what to build.
The whole framework rests on one idea: types are the contract. Once you see that, validation, docs, serialization, and DI all stop being separate features and become one coherent thing.