Python asyncio force timeout

Question

Using asyncio a coroutine can be executed with a timeout so it gets cancelled after the timeout:

@asyncio.coroutine
def coro():
    yield from asyncio.sleep(10)

loop = asyncio.get_event_loop()
loop.run_until_complete(asyncio.wait_for(coro(), 5))

The above example works as expected (it times out after 5 seconds).

However, when the coroutine doesn't use asyncio.sleep() (or other asyncio coroutines) it doesn't seem to time out. Example:

@asyncio.coroutine
def coro():
    import time
    time.sleep(10)

loop = asyncio.get_event_loop()
loop.run_until_complete(asyncio.wait_for(coro(), 1))

This takes more than 10 seconds to run because the time.sleep(10) isn't cancelled. Is it possible to enforce the cancellation of the coroutine in such a case?

If asyncio should be used to solve this, how could I do that?

Answer 1

No, you can't interrupt a coroutine unless it yields control back to the event loop, which means it needs to be inside a yield from call. asyncio is single-threaded, so when you're blocking on the time.sleep(10) call in your second example, there's no way for the event loop to run. That means when the timeout you set using wait_for expires, the event loop won't be able to take action on it. The event loop doesn't get an opportunity to run again until coro exits, at which point its too late.

This is why in general, you should always avoid any blocking calls that aren't asynchronous; any time a call blocks without yielding to the event loop, nothing else in your program can execute, which is probably not what you want. If you really need to do a long, blocking operation, you should try to use BaseEventLoop.run_in_executor to run it in a thread or process pool, which will avoid blocking the event loop:

import asyncio
import time
from concurrent.futures import ProcessPoolExecutor

@asyncio.coroutine
def coro(loop):
    ex = ProcessPoolExecutor(2)
    yield from loop.run_in_executor(ex, time.sleep, 10)  # This can be interrupted.

loop = asyncio.get_event_loop()
loop.run_until_complete(asyncio.wait_for(coro(loop), 1))

Answer 2

Thx @dano for your answer. If running a coroutine is not a hard requirement, here is a reworked, more compact version

import asyncio, time

timeout = 0.5
loop = asyncio.get_event_loop()
future = asyncio.wait_for(loop.run_in_executor(None, time.sleep, 2), timeout)
try:
    loop.run_until_complete(future)
    print('Thx for letting me sleep')
except asyncio.exceptions.TimeoutError:
    print('I need more sleep !')

For the curious, a little debugging in my Python 3.8.2 showed that passing None as an executor results in the creation of a _default_executor, as follows:

self._default_executor = concurrent.futures.ThreadPoolExecutor()