TL;DR: Install FastAPI and Uvicorn, define your endpoints with Python decorators, and run the server with a single command. FastAPI auto-generates interactive docs, making it one of the fastest ways to build a production-ready REST API.
Step 1: Set Up Your Environment
Create a project folder and a virtual environment to keep dependencies isolated. Run python -m venv venv, activate it, then install the essentials with pip install fastapi uvicorn. FastAPI handles routing and validation, while Uvicorn serves the app.
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Step 2: Create Your First App
Create a file named main.py and import FastAPI. Instantiate the app with app = FastAPI(). This object is the core of your API—every route, middleware, and dependency attaches to it.
Step 3: Define a GET Endpoint
Add a root route using a decorator: @app.get("/"). Below it, define a function that returns a dictionary like {"message": "Hello World"}. FastAPI automatically converts the dict to JSON. Run uvicorn main:app --reload and visit http://127.0.0.1:8000 to see the response.
Step 4: Add Path and Query Parameters
Path parameters go directly in the URL: @app.get("/items/{item_id}") with a function accepting item_id: int. FastAPI validates the type automatically and returns a 422 error for bad input. Query parameters work the same way—just add them as function arguments with defaults, like skip: int = 0.
Step 5: Handle POST Requests with Pydantic
Define a Pydantic model class inheriting from BaseModel to describe your request body. For example, a Item with name: str and price: float. Then use @app.post("/items") and accept the model as a parameter. FastAPI validates incoming JSON against the model and rejects malformed data before your code runs.
Step 6: Organize with Routers
As your API grows, split endpoints into modules using APIRouter. Create a router in a separate file, define routes on it, then include it in main.py with app.include_router(). This keeps code maintainable and mirrors real-world project structure.
Step 7: Test and Explore
FastAPI generates Swagger UI at /docs and ReDoc at /redoc. Use these interactive pages to test endpoints without writing client code. For automated testing, install pytest and httpx, then use FastAPI’s TestClient to simulate requests.
Pro Tips
Use response models to hide sensitive fields, add status codes explicitly, and leverage dependency injection for shared logic like database sessions. Always run with --reload during development but disable it in production.
FAQ
Q: Do I need to know async Python to use FastAPI?
A: No. Regular def functions work fine, but using async def improves performance for I/O-bound tasks like database calls.
Q: How do I connect a database?
A: Use SQLAlchemy or SQLModel with dependency injection to provide sessions per request. FastAPI’s docs include full tutorials for both.</
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