TL;DR: Install FastAPI and Uvicorn, then define your routes with Python decorators and Pydantic models for automatic validation. Run the app with Uvicorn, and you get interactive docs and a production-ready REST API with minimal boilerplate.
Step 1: Set Up Your Environment
Create a project folder and a virtual environment to isolate dependencies. 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 the App and First Endpoint
In a file named main.py, import FastAPI and instantiate it: app = FastAPI(). Add a root route using the @app.get("/") decorator above a function that returns a dictionary. FastAPI automatically converts that dictionary to JSON.
Step 3: Add Path and Query Parameters
Define dynamic routes like @app.get("/items/{item_id}") and accept item_id: int in the function signature. FastAPI validates types automatically and returns a 422 error for bad input, saving you manual checks.
Step 4: Use Pydantic Models for POST Requests
Create a class inheriting from BaseModel with typed fields such as name: str and price: float. Use it as a parameter in a @app.post("/items") route. FastAPI parses the JSON body, validates it, and hands you a clean object.
Step 5: Run and Test
Start the server with uvicorn main:app --reload. Visit http://127.0.0.1:8000/docs for Swagger UI, where you can test every endpoint interactively without writing a client.
Tips
Use response_model to control output and hide sensitive fields. Group related routes with APIRouter as your project grows. Add status_code=201 for creations, and handle errors with HTTPException. Always pin dependency versions for reproducible deployments.
FAQ
Q: Do I need to know async Python to use FastAPI?
A: No. Regular def functions work fine, but async def improves performance for I/O-bound tasks like database calls.
Q: How is FastAPI different from Flask?
A: FastAPI includes automatic data validation, type hints, and generated API docs out of the box, while Flask requires extensions for these features.
Q: Can I deploy FastAPI to production easily?
A: Yes. Run it behind Gunicorn with Uvicorn workers, or deploy to platforms like Docker, Railway, or AWS Lambda using Mangum.
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