python - 等待 asyncio.Future 引发 concurrent.futures._base.CancelledError 而不是等待设置值/异常

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)

程序不是 read_future 等待 fut 的结果被设置，而是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 设置值/异常后继续。

Answer 1

你的问题是asyncio.gather is itself async (返回等待)；通过不 await 处理它，您从未将控制权交还给事件循环，也没有存储可等待对象，因此它立即被清理，隐式取消它，并且通过扩展，它的所有可等待对象控制。

要修复，只需确保您await gather 的结果:

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

Try it online!