Python Event类(threading模块下)介绍

Event是threading模块下一个简单的类,主要用于线程间通信。

Event本身管理一个信号标志，信号标志为真时，线程被唤醒，信号标志为假时，通常搭配wait()方法阻塞线程。

Evevt类提供的方法：

• is_set() ：该方法返回Event的信号标志是否为True。
• set() ：将信号标志设置为True，并唤醒所有处于等待状态的线程。
• clear() ：将信号标志设置为False，通常接下来会调用wait()方法阻塞当前线程。
• wait(timeout) ：阻塞当前线程。

示例1：

import threading
from time import sleep
event = threading.Event()
def info():
    print('%s is ready to do.' % (threading.currentThread().name))
    event.wait()
    print('%s is running.' % (threading.currentThread().name))
threading.Thread(target=info,name=('小白')).start()
threading.Thread(target=info,name=('小黑')).start()
sleep(2)
print('%s is running.' % (threading.currentThread().name))
event.set()

运行结果：

小白 is ready to do.
小黑 is ready to do.
MainThread is running.
小白 is running.
小黑 is running.

<分析>

这个例子十分简单，只用到了Event类的wait()方法和set()方法，可以看到两条线程均在event.wait()处被阻塞，之后一直到主线程运行到event.set()处才又被重新唤醒。

示例2：

用Event类实现生产者消费者问题

问题：

假设有一群生产者(Producer)和一群消费者（Consumer）通过一个市场来交互产品。生产者的”策略“是如果市场上剩余的产品少于10个，那么就生产3个产品放到市场上；而消费者的”策略“是如果市场上剩余产品的数量多于7个，那么就消费1个产品。

代码：

import threading
from time import sleep
product = 10
event1 = threading.Event()
event2 = threading.Event()
class Producer(threading.Thread):
    def __init__(self,name):
        super().__init__()
        self.name = name
        self.lock = threading.RLock()
    def run(self):
        global product
        while True:
            if self.lock.acquire():
                if product >= 10:
                    event1.clear()
                    event2.set()
                    event1.wait()
                else:
                    event1.set()
                    product += 3
                    message = self.name + " produced 3 products."
                    print(message)
                self.lock.release()
                sleep(1)

class Consumer(threading.Thread):
    def __init__(self,name):
        super().__init__()
        self.name = name
        self.lock = threading.RLock()
    def run(self):
        global product
        while True:
            if self.lock.acquire():
                if product <= 7:
                    event1.set()
                    event2.clear()
                    event2.wait()
                else:
                    event2.set()
                    product -= 1
                    message = self.name + " consumed 1 products."
                    print(message)
                self.lock.release()
                sleep(1)
def main():
    for i in range(2):
        p = Producer('Producer-%d'%i)
        p.start()
    for i in range(3):
        c = Consumer('Consumer-%d'%i)
        c.start()

if __name__ == '__main__':
    main()             

运行结果：

Consumer-0 consumed 1 products.
Consumer-1 consumed 1 products.
Consumer-2 consumed 1 products.
Producer-1 produced 3 products.
Consumer-0 consumed 1 products.
Consumer-2 consumed 1 products.
Consumer-1 consumed 1 products.
Producer-1 produced 3 products.
Consumer-1 consumed 1 products.
Consumer-2 consumed 1 products.
Consumer-0 consumed 1 products.
Producer-0 produced 3 products.
......

<分析>

这个示例与我上一盘用Condition类实现生产者消费者问题的思路是差不多的，鉴于上一篇已有过详解，这里就略过。无非是用
Event+Lock 代替了 condition 的功能。