Python 与 loop_forever 一起使用期货

Question

刚开始尝试异步，看起来很酷。我正在尝试将 futures 与永远运行的异步协程一起使用，但出现此错误：

Task exception was never retrieved
future: <Task finished coro=<slow_operation() done, defined at ./asynchio-test3.py:5> exception=InvalidStateError("FINISHED: <Future finished result='This is the future!'>",)>

这是我的代码，如果我删除与期货相关的 3 行，它会按预期运行：

import asyncio

@asyncio.coroutine
def slow_operation():
    yield from asyncio.sleep(1)
    print ("This is the task!")
    future.set_result('This is the future!')
    asyncio.async(slow_operation())

def got_result(future):
    print(future.result())

loop = asyncio.get_event_loop()
future = asyncio.Future()
future.add_done_callback(got_result)
asyncio.async(slow_operation())
try:
    loop.run_forever()
finally:
    loop.close()

Answer 1

slow_operator被无限期地调用，多次为同一个未来对象调用set_result；这是不可能的。

>>> import asyncio
>>> future = asyncio.Future()
>>> future.set_result('result')
>>> future.set_result('result')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python35\lib\asyncio\futures.py", line 329, in set_result
    raise InvalidStateError('{}: {!r}'.format(self._state, self))
asyncio.futures.InvalidStateError: FINISHED: <Future finished result='result'>

为每个 slow_operator 调用创建新的未来。例如：

@asyncio.coroutine
def slow_operation(future):
    yield from asyncio.sleep(1)
    print ("This is the task!")
    future.set_result('This is the future!')
    asyncio.async(slow_operation(new_future()))

def got_result(future):
    print(future.result())

def new_future():
    future = asyncio.Future()
    future.add_done_callback(got_result)
    return future

loop = asyncio.get_event_loop()
asyncio.async(slow_operation(new_future()))
try:
    loop.run_forever()
finally:
    loop.close()

顺便说一句，如果您使用的是 Python 3.5+:

，则可以使用新语法 (async, await)

async def slow_operation(future):
    await asyncio.sleep(1)
    print ("This is the task!")
    future.set_result('This is the future!')
    asyncio.ensure_future(slow_operation(new_future()))

Answer 2

按照@falsetru 的回答，这是一个完整的程序，有 3 个异步协程，每个协程都有自己的 got_result 函数。我使用的是 v3.4，所以这就是我不使用新语法的原因。作为一个有趣的副作用，输出清楚地展示了协程的单线程特性。我希望它可以作为某人的模板：

import asyncio

@asyncio.coroutine
def task1(future):
    yield from asyncio.sleep(1)
    print ("This is operation#1")
    future.set_result('This is the result of operation #1!')
    asyncio.async(task1(new_future(got_result1)))

def got_result1(future):
    print(future.result())

@asyncio.coroutine
def task2(future):
    yield from asyncio.sleep(1)
    print ("This is operation#2")
    future.set_result('This is the result of operation #2!')
    asyncio.async(task2(new_future(got_result2)))

def got_result2(future):
    print(future.result())

@asyncio.coroutine
def task3(future):
    yield from asyncio.sleep(1)
    print ("This is operation#3")
    future.set_result('This is the result of operation #3!')
    asyncio.async(task3(new_future(got_result3)))

def got_result3(future):
    print(future.result())

def new_future(callback):
    future = asyncio.Future()
    future.add_done_callback(callback)
    return future

tasks = [task1(new_future(got_result1)),
        task2(new_future(got_result2)),
        task3(new_future(got_result3))]

loop = asyncio.get_event_loop()
for task in tasks:
    asyncio.async(task)

try:
    loop.run_forever()
finally:
    loop.close()

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