asyncio catch TimeoutError in Task

Question

我有一个asyncio.Task，我需要在一段时间后取消。在取消之前，任务需要执行一些清理。根据文档，我应该能够调用 task.cancel 或asyncio.wait_for(coroutine, delay)并在协程中拦截asyncio.TimeoutError，但以下示例不起作用。我尝试拦截其他错误，并改为调用task.cancel，但都没有奏效。我是否误解了取消任务的工作原理？

@asyncio.coroutine
def toTimeout():
  try:
    i = 0
    while True:
      print("iteration ", i, "......"); i += 1
      yield from asyncio.sleep(1)
  except asyncio.TimeoutError:
    print("timed out")
def main():
  #... do some stuff
  yield from asyncio.wait_for(toTimeout(), 10)
  #... do some more stuff
asyncio.get_event_loop().run_until_complete(main())
asyncio.get_event_loop().run_forever()

Answer 1

asyncio.wait_for的文档指定它将取消基础任务，然后从wait_for调用本身引发TimeoutError：

返回未来或协程的结果。发生超时时，它 取消任务并引发asyncio.TimeoutError。

您是对的，任务取消确实可以被拦截：

[ Task.cancel ] 安排将CancelledError扔进包装中 通过事件循环的下一个周期的协程。然后协程 有机会使用 try/except/finally .

请注意，文档指定将CancelledError抛入协程，而不是TimeoutError 。

如果您进行这种调整，事情将按预期方式进行：

import asyncio
@asyncio.coroutine
def toTimeout():
  try:
    i = 0
    while True:
      print("iteration ", i, "......"); i += 1
      yield from asyncio.sleep(1)
  except asyncio.CancelledError:
    print("timed out")
def main():
  #... do some stuff
  yield from asyncio.wait_for(toTimeout(), 3)
  #... do some more stuff
asyncio.get_event_loop().run_until_complete(main())

输出：

iteration  0 ......
iteration  1 ......
iteration  2 ......
timed out
Traceback (most recent call last):
  File "aio.py", line 18, in <module>
    asyncio.get_event_loop().run_until_complete(main())
  File "/usr/lib/python3.4/asyncio/base_events.py", line 316, in run_until_complete
    return future.result()
  File "/usr/lib/python3.4/asyncio/futures.py", line 275, in result
    raise self._exception
  File "/usr/lib/python3.4/asyncio/tasks.py", line 238, in _step
    result = next(coro)
  File "aio.py", line 15, in main
    yield from asyncio.wait_for(toTimeout(), 3)
  File "/usr/lib/python3.4/asyncio/tasks.py", line 381, in wait_for
    raise futures.TimeoutError()
concurrent.futures._base.TimeoutError

如您所见，现在'timed out'在wait_for提高TimeoutError之前打印。