Async & Sync Handlers¶

Cello is built on top of an ASGI foundation, which means it can handle both regular def handlers and async def handlers without any extra configuration. This example shows both styles running in the same application, proving they coexist seamlessly.

Features Demonstrated¶

Defining a plain synchronous def route handler
Defining a coroutine async def route handler with await
Mixing sync and async handlers freely within one App instance
Using asyncio.sleep() to simulate non-blocking I/O (e.g., database queries)

Complete Source Code¶

"""Test async and sync handlers work correctly."""
import asyncio
from cello import App

app = App()

@app.get("/sync")
def sync_handler(request):
    return {"type": "sync", "message": "Hello from sync handler!"}

@app.get("/async")
async def async_handler(request):
    await asyncio.sleep(0.1)
    return {"type": "async", "message": "Hello from async handler!"}

@app.get("/users/{id}")
async def get_user(request):
    user_id = request.params["id"]
    await asyncio.sleep(0.05)
    return {"id": user_id, "name": f"User {user_id}"}

if __name__ == "__main__":
    app.run()

Running This Example¶

python examples/async_demo.py
# then test it:
curl http://127.0.0.1:8000/sync
curl http://127.0.0.1:8000/async
curl http://127.0.0.1:8000/users/42

Key Concepts¶

ASGI under the hood — Cello's server loop runs inside an event loop, so async def handlers are awaited natively without any thread-pool overhead.
Sync handler wrapping — Plain def handlers are automatically run in a thread executor so they never block the event loop, keeping the server responsive.
await asyncio.sleep() — A lightweight stand-in for real async I/O (database calls, HTTP requests); replace with actual await-able calls in production.
Transparent path params — request.params["id"] works identically in both sync and async handlers — no API differences between the two styles.