asyncio：适用于 Python 3.10，但不适用于 Python 3.8

正如在this answer中所讨论的，Python3.10之前的信号量基于当前运行的循环将其循环设置为__init__，而asyncio.run则开始一个新的循环.因此，当您try 和您的CORO async.run时，您使用的循环与您的Semaphore定义的循环不同，其正确的错误消息实际上是 got Future <Future pending> attached to a different loop.

幸运的是，让代码在两个版本上运行并不太难:

Solution 1个

不要创建新的循环，使用现有的循环运行您的函数:

import asyncio

sem: asyncio.Semaphore = asyncio.Semaphore(value=2)


async def async_task() -> None:
    async with sem:
        await asyncio.sleep(1)
        print(f"spam {sem._value}")

async def async_run() -> None:
    await asyncio.gather(*[async_task() for _ in range(3)])

loop = asyncio.get_event_loop()
loop.run_until_complete(async_run())
loop.close()

Solution 2个

在由asyncio.run创建的循环中初始化信号量对象:

import asyncio

async def async_task2(sem) -> None:
    async with sem:
        await asyncio.sleep(1)
        print(f"spam {sem._value}")

async def async_run2() -> None:
    sem = asyncio.Semaphore(2)
    await asyncio.gather(*[async_task2(sem) for _ in range(3)])


asyncio.run(async_run2())

这两个代码片段都可以在python3.8和python3.10上运行.大概就是因为这样的怪异，他们才有了removed the loop parameter from most of asyncio in python 3.10.

比较信号量从3.8到3.10的__init__:

Python 3.8个

class Semaphore(_ContextManagerMixin):
    """A Semaphore implementation.
    A semaphore manages an internal counter which is decremented by each
    acquire() call and incremented by each release() call. The counter
    can never go below zero; when acquire() finds that it is zero, it blocks,
    waiting until some other thread calls release().
    Semaphores also support the context management protocol.
    The optional argument gives the initial value for the internal
    counter; it defaults to 1. If the value given is less than 0,
    ValueError is raised.
    """

    def __init__(self, value=1, *, loop=None):
        if value < 0:
            raise ValueError("Semaphore initial value must be >= 0")
        self._value = value
        self._waiters = collections.deque()
        if loop is None:
            self._loop = events.get_event_loop()
        else:
            self._loop = loop
            warnings.warn("The loop argument is deprecated since Python 3.8, "
                          "and scheduled for removal in Python 3.10.",
                          DeprecationWarning, stacklevel=2)

Python 3.10:

class Semaphore(_ContextManagerMixin, mixins._LoopBoundMixin):
    """A Semaphore implementation.
    A semaphore manages an internal counter which is decremented by each
    acquire() call and incremented by each release() call. The counter
    can never go below zero; when acquire() finds that it is zero, it blocks,
    waiting until some other thread calls release().
    Semaphores also support the context management protocol.
    The optional argument gives the initial value for the internal
    counter; it defaults to 1. If the value given is less than 0,
    ValueError is raised.
    """

    def __init__(self, value=1, *, loop=mixins._marker):
        super().__init__(loop=loop)
        if value < 0:
            raise ValueError("Semaphore initial value must be >= 0")
        self._value = value
        self._waiters = collections.deque()
        self._wakeup_scheduled = False