为什么 await 不等待 asyncio.create_subprocess_exec()

Question

我正在编写一个协程来根据教程在 python 中执行 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()

上面这段代码工作得很好。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():

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

脚本未按预期运行：

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 or result = yield from future – suspends the coroutine until the future is done, then returns the future’s result, or raises an exception, which will be propagated. (If the future is cancelled, it will raise a CancelledError exception.) Note that tasks are futures, and everything said about futures also applies to tasks.

result = await coroutine or result = yield from coroutine – wait for another coroutine to produce a result (or raise an exception, which will be propagated). The coroutine expression must be a call to another coroutine.

return expression – produce a result to the coroutine that is waiting for this one using await or yield from.

raise exception – raise an exception in the coroutine that is waiting for this one using await or yield from.

当协程 suspending 和 waiting 时，进程的实际流程是什么？

这里是asyncio.create_subprocess_exec()的源代码，asyncio.sleep()是协程。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()

Answer 1

您正在等待进程启动。你没有等它完成。 await process.wait() 等待它完成。

为什么 await 不等待 asyncio.create_subprocess_exec()

Why await doesn't wait asyncio.create_subprocess_exec()

python

asynchronous

coroutine

python-asyncio