Timeouts and cancellation

Deciding when to stop waiting

A request with no timeout can hang forever, and one hung request can hold up everything behind it. asyncio.wait_for puts a limit on any awaitable:

try:
    result = await asyncio.wait_for(slow_thing(), timeout=2.0)
except asyncio.TimeoutError:
    result = None

When the timeout fires, the coroutine is cancelled: asyncio.CancelledError is raised inside it at its current await point. That is worth understanding, because it means cleanup still runs:

try:
    await something()
except asyncio.CancelledError:
    await tidy_up()
    raise            # re-raise; swallowing a cancellation is almost always wrong

Re-raising matters. A coroutine that catches CancelledError and returns normally has lied to whoever cancelled it.

Your turn: write with_timeout(seconds) that awaits a slow operation and returns "finished" or "timed out".