Timeouts, cancellation, shield

Timeouts and cancellation, done properly

Cancellation in asyncio is cooperative and exception-based: cancelling a task arranges CancelledError to be raised at its current await point.

asyncio.timeout(seconds) (3.11+) wraps a block:

import asyncio

async def slow_api():
    await asyncio.sleep(10)

async def handler():
    try:
        async with asyncio.timeout(0.5):
            await slow_api()
    except TimeoutError:
        return "gave up"          # TimeoutError in 3.11+, asyncio.TimeoutError before

What professionals must know:

• Timeouts cancel; cancellation propagates. Inner finally blocks and async context managers still run — cleanup code must be cancellation-safe (no unbounded awaits inside cleanup).
• A coroutine may shield a critical section with asyncio.shield(coro) — the shielded await still gets CancelledError in the caller, but the inner operation continues.
• Swallowing CancelledError (bare except Exception does NOT catch it in 3.8+... but except BaseException does) is the classic way to break structured concurrency. Only catch it to re-raise after cleanup.
• asyncio.call_later, loop-bound timers, and asyncio.to_thread (3.9+) for offloading blocking calls to a thread round out the toolkit.