信号灯

import threading,time
import random
def light():     ##灯不会应为car而做出任何改变,而是车更加灯做出改变。红灯听
    if not event.isSet():
        event.set() #wait就不阻塞 #绿灯状态
    count = 0
    while True:
        if count < 10:   ##10 以下时绿灯
            print '33[42;1m--green light on---33[0m'
        elif count <13: ## 10 - 12 秒时黄灯
            print '33[43;1m--yellow light on---33[0m'
        elif count <20:   ## 13--19 秒的时候时红灯
            if event.isSet():   ## 判断event.isSet() 如果为真,也就是如果为绿灯,就event.clear(),让它变成红灯
                event.clear() #clear the green light , switch on the red light
            print '33[41;1m--red light on---33[0m'
        else:
            count = 0
            event.set()   #打开绿灯
        time.sleep(1)
        count +=1


def car(n):
    while 1:
        time.sleep(random.randrange(3))
        if  event.isSet(): #绿灯
            print "car [%s] is running.." % n
        else:# red light
            print "car [%s] is waiting for the red light.." %n
            event.wait()
            print "Green light is on , car %s is running.............." %n



if __name__ == '__main__':
    event = threading.Event()
    Light = threading.Thread(target=light)    ##启动信号等线程
    Light.start()
    for i in range(3):
        t = threading.Thread(target=car,args=(i,))    ##启动车线程
        t.start()

'''
Events #

An event is a simple synchronization object;
the event represents an internal flag, and threads
can wait for the flag to be set, or set or clear the flag themselves.

event = threading.Event()

# a client thread can wait for the flag to be set
event.wait()     ##等待标志位被设置,如果标志位没有被设置,就阻塞,一直等到标志位被设置才能继续走下去

# a server thread can set or reset it
event.set()     ## 设置标志位
event.clear()   ##清除标志位
If the flag is set, the wait method doesn’t do anything.
If the flag is cleared, wait will block until it becomes set again.
Any number of threads may wait for the same event.
'''