Two async operations with one timeout

Question

Basically I want:

await action1()
await action2()
return result

with a single timeout for both actions and - that's important - with an error message telling which action timed out.

For comparison, with just one action:

try:
    await asyncio.wait_for(action(), timeout=1.0)
except asyncio.TimeoutError:
    raise RuntimeError("Problem")

Now with two actions I have this and don't like it.

import asyncio

async def a2():
    try:
        await asyncio.sleep(1.0)
    except asyncio.CancelledError:
        raise RuntimeError("Problem 1") from None
    try:
        await asyncio.sleep(1.0)
    except asyncio.CancelledError:
        raise RuntimeError("Problem 2") from None
    return True


async def test():
    loop = asyncio.get_event_loop()
    action_task = loop.create_task(a2())
    # timeouts: 0.5 -> Problem1 exc; 1.5 -> Problem2 exc; 2.5 -> OK
    try:
        await asyncio.wait_for(action_task, timeout=0.5)
    except asyncio.TimeoutError:
        pass
    result = await action_task

asyncio.get_event_loop().run_until_complete(test())

I find it really counter-intuitive to have:

except asyncio.TimeoutError:
     pass

where timeout handling is the main functionality. Can you suggest a better way?

Answer 1

Can you suggest a better way?

Your code is correct, but if you're looking for something more elegant, perhaps a context manager would fit your usage:

class Timeout:
    def __init__(self, tmout):
        self.tmout = tmout
        self._timed_out = False
        self._section = None

    async def __aenter__(self):
        loop = asyncio.get_event_loop()
        self._timer = loop.call_later(self.tmout, self._cancel_task,
                                      asyncio.current_task())
        return self

    def set_section(self, section):
        self._section = section

    def _cancel_task(self, task):
        self._timed_out = True
        task.cancel()

    async def __aexit__(self, t, v, tb):
        if self._timed_out:
            assert t is asyncio.CancelledError
            raise RuntimeError(self._section) from None
        else:
            self._timer.cancel()

One would use it as follows:

async def main():
    # raises RuntimeError("second sleep")
    async with Timeout(1) as tmout:
        tmout.set_section("first sleep")
        # increase above 1.0 and "first sleep" is raised
        await asyncio.sleep(0.8)
        tmout.set_section("second sleep")
        await asyncio.sleep(0.5)

asyncio.get_event_loop().run_until_complete(main())