Python多线程之Threading.Event

　　多线程之间的通信在任何语言一直是个难点。Python提供了非常简单的通信机制 Threading.Event,通用的条件变量。多个线程可以等待某个事件的发生，在事件发生后，所有的线程都会被激活。

　　Threading.Event 官方解释:

      "

This is one of the simplest mechanisms for communication between threads: one thread signals an event and other threads wait for it.

An event object manages an internal flag that can be set to true with the  set() method and reset to false with the clear() method. The wait() method blocks until the flag is true.

"

    我自己的理解就是 Event是Python多线程通信的最简单的机制之一.一个线程标识一个事件,其他线程一直处于等待状态。

    一个事件对象管理一个内部标示符,这个标示符可以通过set()方法设为True,通过clear()方法重新设为False,wait()方法则使线程一直处于阻塞状态,直到标示符变为True

   也就是说我们可以通过 以上三种方法来多个控制线程的行为。

   下面一个简单的例子,启动三个子线程,并让他们一直处于等待状态,在主线程睡眠5秒后唤醒他们

import threading
import time

class TestThread(threading.Thread):
    def __init__(self, name, event):
        super(TestThread, self).__init__()
        self.name = name
        self.event = event
    
    def run(self):
        print 'Thread: ', self.name, ' start at:', time.ctime(time.time())
        self.event.wait()
        print 'Thread: ', self.name, ' finish at:', time.ctime(time.time())
        
def main():
    event = threading.Event()
    threads = []
    for i in range(1, 5):
        threads.append(TestThread(str(i), event))
    print 'main thread start at: ', time.ctime(time.time())
    event.clear()
    for thread in threads:
        thread.start()
    print 'sleep 5 seconds.......'
    time.sleep(5)
    print 'now awake other threads....'
    event.set()   

main()
    

  在列举一个交通灯例子:

__author__ = 'gongxingfa'
import threading
import random
import time


class VehicleThread(threading.Thread):
    """Class representing a motor vehicle at an intersection"""

    def __init__(self, threadName, event):
        """Initializes thread"""

        threading.Thread.__init__(self, name=threadName)

        # ensures that each vehicle waits for a green light
        self.threadEvent = event

    def run(self):
        """Vehicle waits unless/until light is green"""

        # stagger arrival times
        time.sleep(random.randrange(1, 10))

        # prints arrival time of car at intersection
        print "%s arrived at %s" % \
              (self.getName(), time.ctime(time.time()))

        # wait for green light
        self.threadEvent.wait()

        # displays time that car departs intersection
        print "%s passes through intersection at %s" % \
              (self.getName(), time.ctime(time.time()))


greenLight = threading.Event()
vehicleThreads = []

# creates and starts ten Vehicle threads
for i in range(1, 11):
    vehicleThreads.append(VehicleThread("Vehicle" + str(i),
                                        greenLight))

for vehicle in vehicleThreads:
    vehicle.start()

while threading.activeCount() > 1:
    # sets the Event's flag to false -- block all incoming vehicles
    greenLight.clear()
    print "RED LIGHT! at", time.ctime(time.time())
    time.sleep(3)

    # sets the Event's flag to true -- awaken all waiting vehicles
    print "GREEN LIGHT! at", time.ctime(time.time())
    greenLight.set()
    time.sleep(1)

     从上面的例子看出 threading.Event  相当于贯穿多个线程之间的条件变量,用它能够很方便的来控制多个线程的行为。