为什么我在 Windows 上使用 async 和 await 时会收到 NotImplementedError？

Question

我有这个代码：

import os
import time
import asyncio


async def run_command(*args):
    """
    Example from:
        http://asyncio.readthedocs.io/en/latest/subprocess.html
    """
    # Create subprocess
    process = await asyncio.create_subprocess_exec(
        *args,
        # stdout must a pipe to be accessible as process.stdout
        stdout=asyncio.subprocess.PIPE)

    # Wait for the subprocess to finish
    stdout, stderr = await process.communicate()

    # Result
    result = stdout.decode().strip()

    # Return stdout
    return result


def run_asyncio_commands(tasks):
    """Run tasks using asyncio and return results"""
    loop = asyncio.get_event_loop()
    commands = asyncio.gather(*tasks)  # Unpack list using *
    results = loop.run_until_complete(commands)
    loop.close()
    return results


if __name__ == '__main__':

    start = time.time()

    cmds = [
        ['du', '-sh', '/Users/fredrik/Desktop'],
        ['du', '-sh', '/Users/fredrik'],
        ['du', '-sh', '/Users/fredrik/Pictures']
    ]

    tasks = []
    for cmd in cmds:
        tasks.append(run_command(*cmd))
    results = run_asyncio_commands(tasks)
    print(results)

    end = time.time()
    print('Script ran in', str(end - start), 'seconds')

当我运行在 mac 上 Python 3.6.1 中的那个代码时，我得到这个：

['780K\t/Users/fredrik/Desktop', '46G\t/Users/fredrik', '52M\t/Users/fredrik/Pictures']
Script ran in 6.405519008636475 seconds

但是当我运行在 Windows 上使用相同的脚本时（但是用 du 命令替换了在 Windows 上工作的东西），还有 Python 3.6.1，我明白了：

Traceback (most recent call last):
  File "C:\Users\iruser\Desktop\asynciotest.py", line 66, in <module>
    results = run_asyncio_commands(tasks)
  File "C:\Users\iruser\Desktop\asynciotest.py", line 41, in run_asyncio_commands
    results = loop.run_until_complete(commands)
  File "C:\Users\fredrik\condaenvs\dev_py36\lib\asyncio\base_events.py", line 466, in run_until_complete
    return future.result()
  File "C:\Users\iruser\Desktop\asynciotest.py", line 16, in run_command
    stdout=asyncio.subprocess.PIPE)
  File "C:\Users\fredrik\condaenvs\dev_py36\lib\asyncio\subprocess.py", line 225, in create_subprocess_exec
    stderr=stderr, **kwds)
  File "C:\Users\fredrik\condaenvs\dev_py36\lib\asyncio\base_events.py", line 1190, in subprocess_exec
    bufsize, **kwargs)
  File "C:\Users\fredrik\condaenvs\dev_py36\lib\asyncio\coroutines.py", line 210, in coro
    res = func(*args, **kw)
  File "C:\Users\fredrik\condaenvs\dev_py36\lib\asyncio\base_events.py", line 340, in _make_subprocess_transp
ort
    raise NotImplementedError
NotImplementedError

这是我用 Windows:

替换 Unix 命令的内容

cmds = [['C:/Windows/system32/HOSTNAME.EXE']]

Windows 版本信息：

Python 3.6.1 | packaged by conda-forge | (default, May 23 2017, 14:21:39) [MSC v.1900 64 bit (AMD64)] on win32
Windows 10 Pro, version 1703, OS build 15063.413

Answer 1

不同的事件循环实现方式不同。其中一些有限制（有时 OS 相关）。默认情况下，Windows 使用 SelectorEventLoop and as you can see in doc:

SelectorEventLoop has the following limitations:

SelectSelector is used to wait on socket events: it supports sockets and is limited to 512 sockets.

loop.add_reader() and loop.add_writer() only accept socket handles (e.g. pipe file descriptors are not supported).

Pipes are not supported, so the loop.connect_read_pipe() and loop.connect_write_pipe() methods are not implemented.

Subprocesses are not supported, i.e. loop.subprocess_exec() and loop.subprocess_shell() methods are not implemented.

要运行您在 Windows 中的代码，您可以使用默认可用的替代事件循环 - ProactorEventLoop.

替换行：

loop = asyncio.get_event_loop()

有了这个：

loop = asyncio.ProactorEventLoop()
asyncio.set_event_loop(loop)

您的代码将有效。

Answer 2

3.7.0 Python 文档在此处处理此问题： https://docs.python.org/3/library/asyncio-platforms.html#asyncio-windows-subprocess

如果您使用 Windows，请设置事件循环策略 - 那么您的代码将起作用。

在您的启动中，更改特定于 unix 的部分：

cmds = [
    ['du', '-sh', '/Users/fredrik/Desktop'],
    ['du', '-sh', '/Users/fredrik'],
    ['du', '-sh', '/Users/fredrik/Pictures']
]

处理 Windows & Unix:

if 'win32' in sys.platform:
    # Windows specific event-loop policy & cmd
    asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())
    cmds = [['C:/Windows/system32/HOSTNAME.EXE']]
else:
    # Unix default event-loop policy & cmds
    cmds = [
        ['du', '-sh', '/Users/fredrik/Desktop'],
        ['du', '-sh', '/Users/fredrik'],
        ['du', '-sh', '/Users/fredrik/Pictures']
    ]

为什么我在 Windows 上使用 async 和 await 时会收到 NotImplementedError？

Why am I getting NotImplementedError with async and await on Windows?

python

python-asyncio