python - 在 Python 中使用带有 asyncio 的信号量

Question

我试图限制使用信号量运行的同时异步函数的数量，但我无法让它工作。我的代码归结为：

import asyncio


async def send(i):

    print(f"starting {i}")
    await asyncio.sleep(4)
    print(f"ending {i}")


async def helper():
    async with asyncio.Semaphore(value=5):
        await asyncio.gather(*[
            send(1),
            send(2),
            send(3),
            send(4),
            send(5),
            send(6),
            send(7),
            send(8),
            send(9),
            send(10),
        ])


if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(helper())
    loop.close()

输出是：

starting 1
starting 2
starting 3
starting 4
starting 5
starting 6
starting 7
starting 8
starting 9
starting 10
ending 1
ending 2
ending 3
ending 4
ending 5
ending 6
ending 7
ending 8
ending 9
ending 10

我希望并期望只有 5 个会同时运行，但是所有 10 个会同时启动和停止。我究竟做错了什么？

标签： pythonpython-asynciosemaphore

Answer 1

请在下面找到工作示例，请随时提出问题：

import asyncio


async def send(i: int, semaphore: asyncio.Semaphore):
    # to demonstrate that all tasks start nearly together
    print(f"Hello: {i}")
    # only two tasks can run code inside the block below simultaneously
    async with semaphore:
        print(f"starting {i}")
        await asyncio.sleep(4)
        print(f"ending {i}")


async def async_main():
    s = asyncio.Semaphore(value=2)
    await asyncio.gather(*[send(i, semaphore=s) for i in range(1, 11)])


if __name__ == "__main__":
    loop = asyncio.get_event_loop()
    loop.run_until_complete(async_main())
    loop.close()