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为什么await不等待asyncio.create_subprocess_exec()

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我正在编写一个协程来在python基础上执行shell命令 . 这是基本的:

import asyncio

async def async_procedure():
    process = await asyncio.create_subprocess_exec('ping', '-c', '2', 'google.com')
    await process.wait()
    print('async procedure done.')

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

上面的代码完美无缺 . 它给出了这样的结果:

PING google.com (...) 56(84) bytes of data.
64 bytes from ...: icmp_seq=1 ttl=46 time=34.8 ms
64 bytes from ...: icmp_seq=2 ttl=46 time=34.5 ms

--- google.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 3004ms
rtt min/avg/max/mdev = 33.771/34.437/34.881/0.407 ms
Process done!

当我尝试删除process.wait()时:

async def async_procedure():
    await asyncio.create_subprocess_exec('ping', '-c', '2', 'google.com')
    print('async procedure done.')

该脚本无法按预期工作:

Process done! # This line should be lastest line
PING google.com (...) 56(84) bytes of data.
64 bytes from ...: icmp_seq=1 ttl=46 time=21.1 ms
64 bytes from ...: icmp_seq=2 ttl=46 time=21.8 ms

--- google.com ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 21.135/21.469/21.803/0.334 ms

但是在一个非常相似的例子中没有问题:

async def async_procedure():
    await asyncio.sleep(2)
    print('async procedure done')
  • 那么为什么await不等待asyncio.create_subprocess_exec()?

文档(https://docs.python.org/3/library/asyncio-task.html#coroutine)说:

result = await future或result = yield from future - 暂停协程直到将来完成,然后返回未来的结果,或者引发一个异常,它将被传播 . (如果未来被取消,它将引发CancelledError异常 . )注意任务是期货,所有关于期货的说法也适用于任务 . result =等待coroutine或result =来自coroutine的yield - 等待另一个coroutine产生一个结果(或者引发一个异常,它将被传播) . 协同表达必须是对另一个协程的调用 . return表达式 - 使用await或yield from生成一个等待这个结果的协程的结果 . 引发异常 - 在使用await或yield from等待此代码的协同程序中引发异常 .

  • 当coroutine suspendingwaiting 时,实际进程的流程是什么?

以下是asyncio.create_subprocess_exec()的源代码,asyncio.sleep()是coroutine . 它们都是协程:

@coroutine
def create_subprocess_exec(program, *args, stdin=None, stdout=None,
                           stderr=None, loop=None,
                           limit=streams._DEFAULT_LIMIT, **kwds):
    if loop is None:
        loop = events.get_event_loop()
    protocol_factory = lambda: SubprocessStreamProtocol(limit=limit,
                                                        loop=loop)
    transport, protocol = yield from loop.subprocess_exec(
                                            protocol_factory,
                                            program, *args,
                                            stdin=stdin, stdout=stdout,
                                            stderr=stderr, **kwds)
    return Process(transport, protocol, loop)


@coroutine
def sleep(delay, result=None, *, loop=None):
    """Coroutine that completes after a given time (in seconds)."""
    if delay == 0:
        yield
        return result

    if loop is None:
        loop = events.get_event_loop()
    future = loop.create_future()
    h = future._loop.call_later(delay,
                                futures._set_result_unless_cancelled,
                                future, result)
    try:
        return (yield from future)
    finally:
        h.cancel()

1 回答

  • 0

    您等待该过程开始 . 你没等它完成 . await process.wait() 等待它完成 .

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