Question

好吧，我正在研究python文档以研究我的工作。我是python和编程的新手，我也不太了解异步操作之类的编程概念。

我将Fedora 29与Python 3.7.3结合使用来尝试队列和lib asyncio的示例。

按照下面的队列和异步操作示例：

import asyncio
import random
import time


async def worker(name, queue):
    while True:
        # Get a "work item" out of the queue.
        sleep_for = await queue.get()

        # Sleep for the "sleep_for" seconds.
        await asyncio.sleep(sleep_for)

        # Notify the queue that the "work item" has been processed.
        queue.task_done()

        print(f'{name} has slept for {sleep_for:.2f} seconds')


async def main():
    # Create a queue that we will use to store our "workload".
    queue = asyncio.Queue()

    # Generate random timings and put them into the queue.
    total_sleep_time = 0
    for _ in range(20):
        sleep_for = random.uniform(0.05, 1.0)
        total_sleep_time += sleep_for
        queue.put_nowait(sleep_for)

    # Create three worker tasks to process the queue concurrently.
    tasks = []
    for i in range(3):
        task = asyncio.create_task(worker(f'worker-{i}', queue))
        tasks.append(task)

    # Wait until the queue is fully processed.
    started_at = time.monotonic()
    await queue.join()
    total_slept_for = time.monotonic() - started_at

    # Cancel our worker tasks.
    for task in tasks:
        task.cancel()
    # Wait until all worker tasks are cancelled.
    await asyncio.gather(*tasks, return_exceptions=True)

    print('====')
    print(f'3 workers slept in parallel for {total_slept_for:.2f} seconds')
    print(f'total expected sleep time: {total_sleep_time:.2f} seconds')

asyncio.run(main())

为什么在此示例中我需要取消任务？为什么我可以排除这部分代码

# Cancel our worker tasks.
    for task in tasks:
        task.cancel()
    # Wait until all worker tasks are cancelled.
    await asyncio.gather(*tasks, return_exceptions=True)

示例可以正常工作吗？

Answer 1

为什么在此示例中我需要取消任务？

因为否则它们将无限期地挂起，等待队列中永远不会到达的新项目。在该特定示例中，无论如何您都将退出事件循环，因此它们“挂起”没有什么害处，但是如果将其作为实用程序功能的一部分进行操作，则会创建协程泄漏。

换句话说，取消工作人员会告知他们退出，因为他们不再需要他们的服务，并且需要确保与他们相关的资源被释放。

为什么在此队列示例中需要取消任务？

1 个答案: