Your first route — @app.get('/path')

A route is a function FastAPI calls when a matching HTTP request arrives. Decorator specifies method + path.

main.py v3.1

from fastapi import FastAPI

app = FastAPI()

@app.get('/')
async def root():
    return {'message': 'Hello, World!'}

@app.get('/health')
async def health():
    return {'status': 'ok'}

@app.get('/users/{user_id}')
async def get_user(user_id: int):
    return {'user_id': user_id, 'name': f'User {user_id}'}

# Run: uvicorn main:app --reload
# Visit http://localhost:8000/        -> {'message': 'Hello, World!'}
# Visit http://localhost:8000/health   -> {'status': 'ok'}
# Visit http://localhost:8000/users/42 -> {'user_id': 42, 'name': 'User 42'}
# Visit http://localhost:8000/docs     -> auto-generated API docs!

API simulator click Simulate

Click "Simulate" to see what this FastAPI app does.

🧠 How it works

@app.get('/path') registers the function below as the handler for GET requests to /path. Other decorators: @app.post, @app.put, @app.delete, @app.patch. async def — the function can await I/O. If you have no awaits, regular def works too (FastAPI runs it in a threadpool). Returning a dict — FastAPI auto-converts to JSON. {user_id} in the path is a PATH PARAMETER — FastAPI extracts it and passes as a function arg. The type hint (int) VALIDATES the input — /users/abc returns 422 automatically. /docs is auto-generated Swagger UI — every FastAPI app gets it for free.

✏️ Try these changes

Change '/' to '/home'
→ root is now at /home instead of /
Change user_id: int to user_id: str
→ /users/abc now works (no validation)
Add a new route @app.get('/about') that returns {'app': 'My API'}
→ new endpoint
Change the return of /health to {'status': 'ok', 'uptime': 100}
→ more fields in response

Next lab →

Path parameters — {id} becomes a function arg