python多线程间通信机制-event

python学习笔记之多线程间通信-event

线程间的通讯-- event

event是多线程间通信的一种简单机制，一个线程发出event信号，其他线程等待这个信号。

以一个例子来说明，如下：

import threading
import logging
import time
 
logging.basicConfig(level=logging.DEBUG, format='%(asctime)s %(levelname)s [%(threadName)s] %(message)s')
 
 
def worker(event):
    while not event.is_set():
        logging.debug('in worker fun, event is set ? {0}'.format(event.is_set()))
    logging.debug('event is set')
 
 
def set(event):
    time.sleep(1)
    event.set()
    logging.debug('in set fun, event is set ? {0}'.format(event.is_set()))
 
 
if __name__ == '__main__':
    event = threading.Event()
    w = threading.Thread(target=worker, args=(event,), name="worker")
    w.start()
    s = threading.Thread(target=set, args=(event,), name="set")
    s.start()

在上边的代码中定义了两个函数，并在测试代码上启用两个线程分别调用这两个函数，在调用前要初始化event对象。运行上边的代码会发现一直输出2016-08-14 20:36:49,059 DEBUG [worker] in worker fun, event is set ? False,即是调用worker函数的线程在不断执行，1秒后输出了2016-08-14 20:36:49,063 DEBUG [set] in set fun, event is set ? True，紧跟着输出2016-08-14 20:36:49,063 DEBUG [worker] event is set，如下：

2016-08-14 20:36:49,061 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,061 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,061 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,061 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,061 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,062 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,062 DEBUG [worker] in worker fun, event is set ? False
....略.....
2016-08-14 20:36:49,062 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,062 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,062 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,062 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,062 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,062 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,062 DEBUG [worker] in worker fun, event is set ? False
2016-08-14 20:36:49,063 DEBUG [set] in set fun, event is set ? True
2016-08-14 20:36:49,063 DEBUG [worker] event is set

为什么会这样？

因为在测试代码中启动了两个线程分别调用worker和set函数，worker函数判断当event没有被set时打印类似这样日志in worker fun, event is set ? False。而set函数负责将线程内部的标志设置为true，即执行event.set()语句，它将唤醒所有线程并告知event的内部标志已为true，set函数只是sleep了1秒钟，调用woker函数的线程就执行了很多次了。此时执行worker函数的线程收到这个信号后执行到while not event.is_set()语句时，event.is_set()返回了true，所以整个while语句返回False，那while循环中的语句将被跳过，而去执行logging.debug('event is set')。

• event.wait()

wait方法表示阻塞直到线程内部的标志为true，即有线程调用set()方法。以下边的例子：

import threading
import logging
import time
 
logging.basicConfig(level=logging.DEBUG, format='%(asctime)s %(levelname)s [%(threadName)s] %(message)s')
 
 
def worker(event):
    while not event.is_set():
        event.wait()
        logging.debug('in worker fun, event is set ? {0}'.format(event.is_set()))
    logging.debug('event is set')
 
 
def set(event):
    time.sleep(4)
    event.set()
    logging.debug('in set fun, event is set ? {0}'.format(event.is_set()))
 
 
if __name__ == '__main__':
    event = threading.Event()
    w = threading.Thread(target=worker, args=(event,), name="worker")
    w.start()
    s = threading.Thread(target=set, args=(event,), name="set")
    s.start()

执行上边的代码时，w线程会被一直阻塞，直到4秒种之后，set函数执行到event.set()，内部标志被设置成为true，通知其他所有线程，w线程收到信号后不再阻塞。输出如下：

2016-08-14 21:45:26,326 DEBUG [set] in set fun, event is set ? True
2016-08-14 21:45:26,326 DEBUG [worker] in worker fun, event is set ? True
2016-08-14 21:45:26,326 DEBUG [worker] event is set
 
Process finished with exit code 0

wait可以设置超时时间，表示只阻塞线程一定的时间，如果收到了event.set()信号，线程退出，否则线程被阻塞，只是不是一直阻塞，而是按照设定的时间阻塞线程，如下设置wait时间，代码如下：

import threading
import logging
import time
 
logging.basicConfig(level=logging.DEBUG, format='%(asctime)s %(levelname)s [%(threadName)s] %(message)s')
 
 
def worker(event):
    while not event.is_set():
        event.wait(timeout=1)
        logging.debug('in worker fun, event is set ? {0}'.format(event.is_set()))
    logging.debug('event is set')
 
 
def set(event):
    time.sleep(4)
    event.set()
    logging.debug('in set fun, event is set ? {0}'.format(event.is_set()))
 
 
if __name__ == '__main__':
    event = threading.Event()
    w = threading.Thread(target=worker, args=(event,), name="worker")
    w.start()
    s = threading.Thread(target=set, args=(event,), name="set")
    s.start()

当执行上边代码时，每隔一秒执行worker函数，当到第4秒时，event对象被set，w线程收到信号就退出。