为什么 Python 多处理守护进程不打印到标准输出？

Question

我一直在尝试多处理，但遇到了守护进程。

我有一个守护进程和一个非守护进程，守护进程无限期地每隔一秒发出一次输出，而非守护进程在启动时立即打印输出，休眠 3 秒，然后再次打印并返回。

问题是，守护进程的预期输出根本没有出现。

回顾过去关于守护进程的 SO 问题，常见问题似乎是在守护进程之前结束的其他进程，或者需要刷新以显示输出的标准输出。两者都已解决（我认为），但我仍然只看到非守护进程的打印输出。

代码：

from multiprocessing import Process, current_process
import sys
import time

def worker():
    """
    Announce that the process has started, sleep 3 seconds
    then announce that the process is ending.
    """
    name = current_process().name
    print name, 'starting...'
    sys.stdout.flush()

    time.sleep(3)
    print name, 'ending...'
    sys.stdout.flush()

    return


def daemon():
    """
    Announce that the process has started, beep, then beep
    once every second
    """
    name = current_process().name
    print name, 'starting...'
    print 'beep...'
    sys.stdout.flush()

    while True:
        time.sleep(1)
        print 'beep...'
        sys.stdout.flush()


if __name__=='__main__':
    d = Process(target=daemon)
    d.daemon = True
    d.start()

    p = Process(target=worker)
    p.daemon = False
    p.start()

预期输出：

Process-1 starting... # Order here may vary
beep...
Process-2 starting...
beep...
beep...
Process-2 ending... #There may or may not be another beep here

实际生产的产品：

Process-2 starting...
Process-2 ending...

任何关于为什么会发生这种情况的建议将不胜感激。

Answer 1

您可以通过放置打开日志来更清晰地了解事件的顺序

import multiprocessing as mp
logger = mp.log_to_stderr(logging.INFO)

在其他导入语句之后。然后你的程序会产生类似的东西：

[INFO/Process-1] child process calling self.run()
[INFO/MainProcess] process shutting down
Process-1 starting...
beep...
[INFO/Process-2] child process calling self.run()
[INFO/MainProcess] calling terminate() for daemon Process-1
Process-2 starting...
[INFO/MainProcess] calling join() for process Process-2
Process-2 ending...
[INFO/Process-2] process shutting down
[INFO/Process-2] process exiting with exitcode 0
[INFO/MainProcess] calling join() for process Process-1

因此，主进程首先开始关闭，然后终止进程-1，即守护进程。这就是为什么您在 Process-2 继续时看不到任何哔哔声。