Python: 按完成排序任务后获取对原始任务的引用

Question

问题：asyncio.as_completed出结果后，如何获取原始任务的引用？

与这个 C# 问题基本相同，除了 Python：

示例问题：

# Takes a list of WebClient objects,
# calls each one simultaneously,
# and yields the results immediately as they arrive
# to a synchronous caller.

def yieldThingsAsTheyArrive(webClients):

    tasks = []
    for webClient in webClients:
        # This is what we want to get a reference to later:
        task = webClient.fetch_thing()  # start long-running request asynchronously
        tasks.append(task)

    loop = asyncio.get_event_loop()
    for future in asyncio.as_completed(tasks):
        thing = loop.run_until_complete(future)  # since our caller is synchronous, wait until the task completes so we can yield the final result instead of a future
        thing.originalWebClient = ???  # This is where we need a reference to the original webClient
        yield thing

Answer 1

as_completed 的特殊之处在于它既不会像 asyncio.wait, nor their results like asyncio.gather 那样产生期货。相反，它会生成您需要等待的协程（以您喜欢的任何方式）以按完成顺序获得结果。它无法产生您传递给它的期货，因为此时它还不知道接下来将完成哪个传递的期货。

您可以通过在另一个未来包装任务来关联任意数据，其结果是任务对象（您已将数据附加到该对象）。这本质上等同于 does, only without the static-typing ceremony. Taking the setup from ，可运行的示例如下所示：

import asyncio

async def first():
    await asyncio.sleep(5)
    return 'first'

async def second():
    await asyncio.sleep(1)
    return 'second'

async def third():
    await asyncio.sleep(3)
    return 'third'

def ordinary_generator():
    loop = asyncio.get_event_loop()

    wrappers = []
    for idx, coro in enumerate((first(), second(), third())):
        task = loop.create_task(coro)
        task.idx = idx + 1
        # Wrap the task in a future that completes when the 
        # task does, but whose result is the task object itself.
        wrapper = loop.create_future()
        task.add_done_callback(wrapper.set_result)
        wrappers.append(wrapper)

    for x in asyncio.as_completed(wrappers):
        # yield completed tasks
        yield loop.run_until_complete(x)

for task in ordinary_generator():
    print(task.result(), task.idx)

我推荐的另一个选项是用调用 asyncio.wait(return_when=FIRST_COMPLETED) 的循环替换 as_completed 上的迭代。这也将提供完整的期货，但不需要额外的包装，并导致稍微更惯用的异步代码。我们在每个协程上调用 ensure_future 将其转换为 future，向其附加数据，然后才将其传递给 asyncio.wait()。由于 wait returns 那些相同的期货，附加数据在它们上面。

def ordinary_generator():
    loop = asyncio.get_event_loop()

    pending = []
    for idx, coro in enumerate((first(), second(), third())):
        task = loop.create_task(coro)
        task.idx = idx + 1
        pending.append(task)

    while pending:
        done, pending = loop.run_until_complete(asyncio.wait(
            pending, return_when=asyncio.FIRST_COMPLETED))
        for task in done:
            yield task

Python: 按完成排序任务后获取对原始任务的引用

Python: Get reference to original task after ordering tasks by completion

python-asyncio