Python3.x 如何使用 Python asyncio 限制并发

假设我们有很多链接要下载，每个链接可能需要不同的下载时间.我只允许使用最多3个连接进行下载.现在，我想确保使用asyncio有效地完成这项工作.

以下是我试图实现的目标:在任何时间点，try 确保至少有3次下载正在运行.

Connection 1: 1---------7---9---
Connection 2: 2---4----6-----
Connection 3: 3-----5---8-----

数字代表下载链接，连字符代表等待下载.

这是我现在使用的代码

from random import randint
import asyncio

count = 0


async def download(code, permit_download, no_concurrent, downloading_event):
    global count
    downloading_event.set()
    wait_time = randint(1, 3)
    print('downloading {} will take {} second(s)'.format(code, wait_time))
    await asyncio.sleep(wait_time)  # I/O, context will switch to main function
    print('downloaded {}'.format(code))
    count -= 1
    if count < no_concurrent and not permit_download.is_set():
        permit_download.set()


async def main(loop):
    global count
    permit_download = asyncio.Event()
    permit_download.set()
    downloading_event = asyncio.Event()
    no_concurrent = 3
    i = 0
    while i < 9:
        if permit_download.is_set():
            count += 1
            if count >= no_concurrent:
                permit_download.clear()
            loop.create_task(download(i, permit_download, no_concurrent, downloading_event))
            await downloading_event.wait()  # To force context to switch to download function
            downloading_event.clear()
            i += 1
        else:
            await permit_download.wait()
    await asyncio.sleep(9)

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    try:
        loop.run_until_complete(main(loop))
    finally:
        loop.close()

yields 如预期:

downloading 0 will take 2 second(s)
downloading 1 will take 3 second(s)
downloading 2 will take 1 second(s)
downloaded 2
downloading 3 will take 2 second(s)
downloaded 0
downloading 4 will take 3 second(s)
downloaded 1
downloaded 3
downloading 5 will take 2 second(s)
downloading 6 will take 2 second(s)
downloaded 5
downloaded 6
downloaded 4
downloading 7 will take 1 second(s)
downloading 8 will take 1 second(s)
downloaded 7
downloaded 8

但以下是我的问题:

1. 目前，我只需等待9秒钟，以保持主功能运行，直到下载完成.在退出main功能之前，是否有一种有效的方法来等待最后一次下载完成？(我知道有asyncio.wait个，但我需要存储所有的任务引用，以便它工作)

2. 什么样的图书馆能完成这种任务？我知道javascript有很多异步库，但是Python呢？

编辑: