python Condition类（锁）

Posted 2021-01-31 大道至简,小而蕴真

Condition（条件变量）通常与一个锁关联。需要在多个Contidion中共享一个锁时，可以传递一个Lock/RLock实例给构造方法，否则它将自己生成一个RLock实例。

不理解锁的，请看上一条随笔。

Condition():

• acquire(): 线程锁
• release(): 释放锁
• wait(timeout): 线程挂起，直到收到一个notify通知或者超时（可选的，浮点数，单位是秒s）才会被唤醒继续运行。wait()必须在已获得Lock前提下才能调用，否则会触发RuntimeError。
• notify(n=1): 通知其他线程，那些挂起的线程接到这个通知之后会开始运行，默认是通知一个正等待该condition的线程,最多则唤醒n个等待的线程。notify()必须在已获得Lock前提下才能调用，否则会触发RuntimeError。notify()不会主动释放Lock。
• notifyAll(): 如果wait状态线程比较多，notifyAll的作用就是通知所有线程

import threading
import time

# 商品
product = None
# 条件变量
con = threading.Condition(threading.Lock())


# 生产者方法
def produce():
    global product

    if con.acquire():
        while True:
            print(‘我执行了，produce‘)
            if product is None:
                product = ‘anything‘
                print(‘produce...‘,product)
                print(‘plock=‘,con)
                # 通知消费者，商品已经生产
                con.notify()

            # 等待通知
            con.wait()
            time.sleep(2)


# 消费者方法
def consume():
    global product

    if con.acquire():
        while True:
            print(‘我执行了，consume‘)
            if product is not None:
                print(‘consume...‘,product)
                print(‘clock=‘,con)
                product = None

                # 通知生产者，商品已经没了
                con.notify()

            # 等待通知
            con.wait()
            time.sleep(2)


t2 = threading.Thread(target=consume)
t2.start()
t1 = threading.Thread(target=produce)
t1.start()

1、生产者消费者模型

import threading
import time

condition = threading.Condition()
products = 0

class Producer(threading.Thread):
    def run(self):
        global products
        while True:
            if condition.acquire():
                if products < 10:
                    products += 1;
                    print "Producer(%s):deliver one, now products:%s" %(self.name, products)
                    condition.notify()#不释放锁定，因此需要下面一句
                    condition.release()
                else:
                    print "Producer(%s):already 10, stop deliver, now products:%s" %(self.name, products)
                    condition.wait();#自动释放锁定
                time.sleep(2)

class Consumer(threading.Thread):
    def run(self):
        global products
        while True:
            if condition.acquire():
                if products > 1:
                    products -= 1
                    print "Consumer(%s):consume one, now products:%s" %(self.name, products)
                    condition.notify()
                    condition.release()
                else:
                    print "Consumer(%s):only 1, stop consume, products:%s" %(self.name, products)
                    condition.wait();
                time.sleep(2)

if __name__ == "__main__":
    for p in range(0, 2):
        p = Producer()
        p.start()

    for c in range(0, 3):
        c = Consumer()
        c.start()

2、生产者消费者模型

import threading
 
alist = None
condition = threading.Condition()
 
def doSet():
    if condition.acquire():
        while alist is None:
            condition.wait()
        for i in range(len(alist))[::-1]:
            alist[i] = 1
        condition.release()
 
def doPrint():
    if condition.acquire():
        while alist is None:
            condition.wait()
        for i in alist:
            print i,
        print
        condition.release()
 
def doCreate():
    global alist
    if condition.acquire():
        if alist is None:
            alist = [0 for i in range(10)]
            condition.notifyAll()
        condition.release()
 
tset = threading.Thread(target=doSet,name=‘tset‘)
tprint = threading.Thread(target=doPrint,name=‘tprint‘)
tcreate = threading.Thread(target=doCreate,name=‘tcreate‘)
tset.start()
tprint.start()
tcreate.start()

3、生产者消费者模型

总结

1、release和wait都有释放锁的作用，不同在于wait后，该子线程就在那里挂起等待，要继续执行，就需要接收到notify或者notifyAll来唤醒线程，而release该线程还能继续执行。

2、notify和notifyAll的区别在于，notify只能唤醒一个wait，而notifyAll能唤起所有wait。