Python使用带有loop_forever的Future

Question

刚开始尝试使用看起来非常酷的asynch。 我试图将期货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!'>",)>

如果删除与期货相关的3行，这是我的代码，它将按预期运行：

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被无限期调用，对同一未来对象多次调用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slow_operator 。 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+，则可以使用新的语法（ 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

在@falsetru回答之后，这是一个完整的程序，具有3个异步协程，每个协程都有自己的got_result函数。 我正在使用v3.4，所以这就是为什么我不使用新语法的原因。 作为一个有趣的副作用，输出清楚地表明了协程的单线程性质。 我希望它作为某人的模板有用：

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()