等待 asyncio.Future 引发 concurrent.futures._base.CancelledError 而不是等待 value/exception 被设置

Question

当我运行以下python代码时：

import asyncio
import logging
logging.basicConfig(level=logging.DEBUG)

async def read_future(fut):
    print(await fut)

async def write_future(fut):
    fut.set_result('My Value')

async def main():
    loop = asyncio.get_running_loop()
    fut = loop.create_future()
    asyncio.gather(read_future(fut), write_future(fut))

asyncio.run(main(), debug=True)

而不是 read_future 等待 fut 的结果被设置，程序崩溃并出现以下错误：

DEBUG:asyncio:Using selector: KqueueSelector
ERROR:asyncio:_GatheringFuture exception was never retrieved
future: <_GatheringFuture finished exception=CancelledError() created at /Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/tasks.py:642>
source_traceback: Object created at (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/runners.py", line 43, in run
    return loop.run_until_complete(main)
  File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/base_events.py", line 566, in run_until_complete
    self.run_forever()
  File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/base_events.py", line 534, in run_forever
    self._run_once()
  File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/base_events.py", line 1763, in _run_once
    handle._run()
  File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/events.py", line 88, in _run
    self._context.run(self._callback, *self._args)
  File "<stdin>", line 4, in main
  File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/tasks.py", line 766, in gather
    outer = _GatheringFuture(children, loop=loop)
  File "/Users/<user>/.pyenv/versions/3.7.4/lib/python3.7/asyncio/tasks.py", line 642, in __init__
    super().__init__(loop=loop)
concurrent.futures._base.CancelledError
DEBUG:asyncio:Close <_UnixSelectorEventLoop running=False closed=False debug=True>

我在这段代码中做错了什么？我希望能够等待 Future fut 并在 Future 设置 value/exception 后继续。

Answer 1

来自https://docs.python.org/3/library/asyncio-future.html#asyncio.Future.result：

result()

Return the result of the Future.

If the Future is done and has a result set by the set_result() method, the result value is returned.

If the Future is done and has an exception set by the set_exception() method, this method raises the exception.

If the Future has been cancelled, this method raises a CancelledError exception.

If the Future’s result isn’t yet available, this method raises a InvalidStateError exception.

^{（加粗。）}

我不确定为什么 Future 被取消了，但这似乎是问题的原因。

Answer 2

您的问题是 asyncio.gather is itself async（returns 等待）；如果不 await 处理它，您就不会将控制权交还给事件循环，也不会存储可等待对象，因此它会立即被清理，隐式取消它，并且通过扩展，它控制的所有可等待对象。

要修复，只需确保 await gather 的结果：

await asyncio.gather(read_future(fut), write_future(fut))

Try it online!

等待 asyncio.Future 引发 concurrent.futures._base.CancelledError 而不是等待 value/exception 被设置

Awaiting an asyncio.Future raises concurrent.futures._base.CancelledError instead of waiting for a value/exception to be set

python

python-3.x

async-await

python-asyncio

python-3.7