python多线程之t.setDaemon(True) 和 t.join()

0.目录

1.参考
2.结论
    (1)通过 t.setDaemon(True) 将子线程设置为守护进程（默认False），主线程代码执行完毕后，python程序退出，无需理会守护子线程的状态。
    (2) t.join() 用于阻塞主线程，可以想象成将某个子线程的执行过程插入(join)到主线程的时间线上，主线程的后续代码延后执行。注意和 t.start() 分开写在两个for循环中。
    (3)第一个for循环同时启动了所有子线程，随后在第二个for循环中执行t.join() ，主线程实际被阻塞的总时长==其中执行时间最长的一个子线程。
3.验证过程

1.参考

C:\Program Files\Anaconda2\Lib\threading.py

    def daemon(self):
        """A boolean value indicating whether this thread is a daemon thread (True) or not (False).

        This must be set before start() is called, otherwise RuntimeError is
        raised. Its initial value is inherited from the creating thread; the
        main thread is not a daemon thread and therefore all threads created in
        the main thread default to daemon = False.

        The entire Python program exits when no alive non-daemon threads are
        left.

        """

    def join(self, timeout=None):
        """Wait until the thread terminates.

        This blocks the calling thread until the thread whose join() method is
        called terminates -- either normally or through an unhandled exception
        or until the optional timeout occurs.

        When the timeout argument is present and not None, it should be a
        floating point number specifying a timeout for the operation in seconds
        (or fractions thereof). As join() always returns None, you must call
        isAlive() after join() to decide whether a timeout happened -- if the
        thread is still alive, the join() call timed out.

        When the timeout argument is not present or None, the operation will
        block until the thread terminates.

        A thread can be join()ed many times.

        join() raises a RuntimeError if an attempt is made to join the current
        thread as that would cause a deadlock. It is also an error to join() a
        thread before it has been started and attempts to do so raises the same
        exception.

        """

2.结论

(1)通过 t.setDaemon(True) 将子线程设置为守护进程（默认False），主线程代码执行完毕后，python程序退出，无需理会守护子线程的状态。

(2) t.join() 用于阻塞主线程，可以想象成将某个子线程的执行过程插入(join)到主线程的时间线上，主线程的后续代码延后执行。注意和 t.start() 分开写在两个for循环中。

(3)第一个for循环同时启动了所有子线程，随后在第二个for循环中执行t.join() ，主线程实际被阻塞的总时长==其中执行时间最长的一个子线程。

3.验证过程

(1)子线程默认为【非守护线程】，主线程代码执行完毕，各子线程继续运行，直到所有【非守护线程】结束，python程序退出。

#!usr/bin/env python
#coding:utf-8
import time
import random
import logging

import thread
import threading
from Queue import Queue

lock = threading.Lock()     #'function-call ownership'
rlock = threading.RLock()   #thread ownership

def get_logger():
    logger = logging.getLogger("threading_example")
    logger.setLevel(logging.DEBUG)

    # fh = logging.FileHandler("d:/threading.log")
    fh = logging.StreamHandler()
    fmt = '%(asctime)s - %(threadName)-10s - %(levelname)s - %(message)s'
    formatter = logging.Formatter(fmt)
    fh.setFormatter(formatter)

    logger.addHandler(fh)
    return logger
logger = get_logger()
#################################

class MyThread(threading.Thread):
    def __init__(self, number):
        threading.Thread.__init__(self)
        self.number = number
    def run(self):
        for i in range(2):
            logger.debug(i)
            time.sleep(self.number)

def main():
    logger.debug('MainThread Start')
    threads = [MyThread(3-i) for i in range(3)]  

    for t in threads:
        t.start()

    logger.debug('MainThread End')

if __name__ == '__main__':
    main()

输出：

2017-08-15 17:04:12,512 - MainThread - DEBUG - MainThread Start
2017-08-15 17:04:12,512 - Thread-1   - DEBUG - 0
2017-08-15 17:04:12,513 - Thread-2   - DEBUG - 0
2017-08-15 17:04:12,513 - Thread-3   - DEBUG - 0
2017-08-15 17:04:12,513 - MainThread - DEBUG - MainThread End
2017-08-15 17:04:13,513 - Thread-3   - DEBUG - 1
2017-08-15 17:04:14,513 - Thread-2   - DEBUG - 1
2017-08-15 17:04:15,513 - Thread-1   - DEBUG - 1

(2) t.setDaemon(True)， 主线程代码执行完毕，直接退出python程序，无需理会【守护】子线程的状态。

def main():
    logger.debug('MainThread Start')
    threads = [MyThread(3-i) for i in range(3)]  

    for t in threads:
        t.setDaemon(True)   #将子线程设置为守护进程
        t.start()

    logger.debug('MainThread End')

输出：

2017-08-15 17:06:20,822 - MainThread - DEBUG - MainThread Start
2017-08-15 17:06:20,822 - Thread-1   - DEBUG - 0
2017-08-15 17:06:20,823 - Thread-2   - DEBUG - 0
2017-08-15 17:06:20,823 - MainThread - DEBUG - MainThread End
2017-08-15 17:06:20,823 - Thread-3   - DEBUG - 0

(3)错误用法： d t1.join()阻塞了主线程的for循环，t1结束后才执行t2.start()...实际就变成了单线程顺序执行。

def main():
    logger.debug('MainThread Start')
    threads = [MyThread(3-i) for i in range(3)]  

    for t in threads:
        t.start()
        t.join()
        logger.debug('{} start() join() activeCount: {}'.format(t.getName(), threading.activeCount()))

    logger.debug('MainThread End')

输出：

2017-08-15 17:17:38,219 - MainThread - DEBUG - MainThread Start
2017-08-15 17:17:38,230 - Thread-1   - DEBUG - 0
2017-08-15 17:17:41,230 - Thread-1   - DEBUG - 1
2017-08-15 17:17:44,232 - MainThread - DEBUG - Thread-1 start() join() activeCount: 1
2017-08-15 17:17:44,232 - Thread-2   - DEBUG - 0
2017-08-15 17:17:46,232 - Thread-2   - DEBUG - 1
2017-08-15 17:17:48,233 - MainThread - DEBUG - Thread-2 start() join() activeCount: 1
2017-08-15 17:17:48,233 - Thread-3   - DEBUG - 0
2017-08-15 17:17:49,234 - Thread-3   - DEBUG - 1
2017-08-15 17:17:50,234 - MainThread - DEBUG - Thread-3 start() join() activeCount: 1
2017-08-15 17:17:50,234 - MainThread - DEBUG - MainThread End

(4)异常用法：只对for循环的最后一个子线程执行了t.join() ，如果该子线程的执行时长不是所有子线程中最久的，可能达不到预期效果。

def main():
    logger.debug('MainThread Start')
    # threads = [MyThread(3-i) for i in range(3)]
    threads = [MyThread((3-i)*2) for i in range(3)]   #修改了等待时间

    for t in threads:
        t.start()

    logger.debug('MainThread ing')
    # for t in threads:
    t.join()
    logger.debug('{} is_alive: {} join() activeCount: {}'.format(t.getName(), t.is_alive(), threading.activeCount()))
    logger.debug('MainThread End')

输出：

2017-08-15 18:18:10,924 - MainThread - DEBUG - MainThread Start
2017-08-15 18:18:10,927 - Thread-1   - DEBUG - 0
2017-08-15 18:18:10,930 - Thread-2   - DEBUG - 0
2017-08-15 18:18:10,931 - Thread-3   - DEBUG - 0
2017-08-15 18:18:10,931 - MainThread - DEBUG - MainThread ing
2017-08-15 18:18:12,931 - Thread-3   - DEBUG - 1
2017-08-15 18:18:14,931 - Thread-2   - DEBUG - 1
2017-08-15 18:18:14,931 - MainThread - DEBUG - Thread-3 is_alive: False join() activeCount: 3
2017-08-15 18:18:14,931 - MainThread - DEBUG - MainThread End
2017-08-15 18:18:16,928 - Thread-1   - DEBUG - 1

(5)正常用法：第二个for循环保证每一个子线程都执行了t.join()， 虽然t1运行结束后才执行t2.join(), 但是第一个for循环已经启动了所有子线程，所以主线程实际被阻塞的总时长==其中执行时间最长的一个子线程。

def main():
    logger.debug('MainThread Start')
    # threads = [MyThread(3-i) for i in range(3)]
    threads = [MyThread((i+1)*2) for i in range(3)]     #修改了等待时间

    for t in threads:
        t.start()

    logger.debug('MainThread ing')
    for t in threads:
        logger.debug('{} is_alive: {} join() activeCount: {}'.format(t.getName(), t.is_alive(), threading.activeCount()))
        t.join()
    logger.debug('MainThread End')

输出：

2017-08-15 17:30:00,499 - MainThread - DEBUG - MainThread Start
2017-08-15 17:30:00,499 - Thread-1   - DEBUG - 0
2017-08-15 17:30:00,500 - Thread-2   - DEBUG - 0
2017-08-15 17:30:00,500 - Thread-3   - DEBUG - 0
2017-08-15 17:30:00,500 - MainThread - DEBUG - MainThread ing
2017-08-15 17:30:00,500 - MainThread - DEBUG - Thread-1 is_alive: True join() activeCount: 4
2017-08-15 17:30:02,500 - Thread-1   - DEBUG - 1
2017-08-15 17:30:04,500 - Thread-2   - DEBUG - 1
2017-08-15 17:30:04,500 - MainThread - DEBUG - Thread-2 is_alive: True join() activeCount: 3
2017-08-15 17:30:06,500 - Thread-3   - DEBUG - 1
2017-08-15 17:30:08,503 - MainThread - DEBUG - Thread-3 is_alive: True join() activeCount: 2
2017-08-15 17:30:12,500 - MainThread - DEBUG - MainThread End