进程终止后Popen对象的returncode为None

Question

我正在使用Popen运行一个进程。我需要等待进程结束。我正在检查进程是否已通过returncode终止。当returncode与None不同时，进程必须已终止。问题是，当print_output是False时，returncode始终是None，即使进程已完成运行（终止）。然而，当print_output是True时，情况并非如此。我正在使用以下代码运行进程：

def run(command, print_output=True):
    # code mostly from: http://sharats.me/the-ever-useful-and-neat-subprocess-module.html
    from subprocess import Popen, PIPE
    from threading import Thread
    from queue import Queue, Empty
    from time import sleep

    io_q = Queue()

    def stream_watcher(identifier, stream):
        for line in stream:
            io_q.put((identifier, line))

        if not stream.closed:
            stream.close()

    with Popen(command, stdout=PIPE, stderr=PIPE, universal_newlines=True) as proc:
        if print_output:

            Thread(target=stream_watcher, name='stdout-watcher', args=('STDOUT', proc.stdout)).start()
            Thread(target=stream_watcher, name='stderr-watcher', args=('STDERR', proc.stderr)).start()

            def printer():
                while True:
                    try:
                        # Block for 1 second.
                        item = io_q.get(True, 1)
                    except Empty:
                        # No output in either streams for a second. Are we done?
                        if proc.poll() is not None:
                            break
                    else:
                        identifier, line = item
                        print(identifier + ':', line, end='')

            Thread(target=printer, name='printer').start()

        while proc.returncode is None:
            sleep(2)
            proc.poll()

        if not proc.returncode == 0:
            raise RuntimeError(
                'The process call "{}" returned with code {}. The return code is not 0, thus an error '
                'occurred.'.format(list(command), proc.returncode))

        return proc.stdout, proc.stderr

有什么可能导致这个问题的线索吗？

编辑：发现了一些很奇怪的东西。我运行以下代码：

run(my_command, True)
print('--------done--------')
run(my_command, False)
print('--------done--------')

即使执行了run(my_command, False)，也不会打印'--------done--------'。

Answer 1

我不知道为什么它不起作用，但我认为这与不关闭溪流有关。以下代码有效：

def run(command, print_output=True):
    from subprocess import Popen, PIPE, STDOUT
    from io import StringIO

    popen = Popen(command, stdout=PIPE, stderr=STDOUT, universal_newlines=True)
    out = StringIO()
    for line in popen.stdout:
        if print_output:
            print(line, end='')
        else:
            out.write(line)

    popen.stdout.close()
    return_code = popen.wait()

    if not return_code == 0:
        raise RuntimeError(
            'The process call "{}" returned with code {}. The return code is not 0, thus an error '
            'occurred.'.format(list(command), return_code))

    stdout_string = out.getvalue()
    out.close()

    return stdout_string

Answer 2

TL；博士

在subprocess.Popen()之后添加popen.wait()

解释部分（种类）

Python运行得太快，子进程结束，但无法读取returncode

（我真的不知道它为什么这么做。欢迎解释）

我为什么用这个：

执行Shell命令并获取返回代码和输出（stdout）

def exec_cmd(cmd):
    pop = subprocess.Popen(shlex.split(cmd), stderr=subprocess.STDOUT, stdout=subprocess.PIPE)
    pop.wait()
    return [pop.returncode, pop.communicate()[0]]

另外：请阅读popen page上的.wait警告