python中正确的生产者和消费者问题

假设有一个公共队列,生产者向队列中写数据,消费者从队列中读数据。当队列中没有任何数据的时候,消费者应该停止运行并等待(wait),而不是继续尝试读取数据而引发读取空队列的异常。而当生产者在队列中加入数据之后,应该有一个渠道去告诉(notify)消费者。然后消费者可以再次从队列中进行读取,而IndexError不再出现。

消费者:

from threading import Condition
 
condition = Condition()
 
class ConsumerThread(Thread):
    def run(self):
        global queue
        while True:
            condition.acquire()
            if not queue:
                print "Nothing in queue, consumer is waiting"
                condition.wait()
                print "Producer added something to queue and notified the consumer"
            num = queue.pop(0)
            print "Consumed", num 
            condition.release()
            time.sleep(random.random())
生产者:
class ProducerThread(Thread):
    def run(self):
        nums = range(5)
        global queue
        while True:
            condition.acquire()
            num = random.choice(nums)
            queue.append(num)
            print "Produced", num 
            condition.notify()
            condition.release()
            time.sleep(random.random())
输出:
Produced 3
Consumed 3
Produced 1
Consumed 1
Produced 4
Consumed 4
Produced 3
Consumed 3
Nothing in queue, consumer is waiting
Produced 2
Producer added something to queue and notified the consumer
Consumed 2
Nothing in queue, consumer is waiting
Produced 2
Producer added something to queue and notified the consumer
Consumed 2
Nothing in queue, consumer is waiting
Produced 3
Producer added something to queue and notified the consumer
Consumed 3
Produced 4
Consumed 4
Produced 1
Consumed 1

解释:

  • 对于消费者,在消费前检查队列是否为空。
  • 如果为空,调用condition实例的wait()方法。
  • 消费者进入wait(),同时释放所持有的lock。
  • 除非被notify,否则它不会运行。
  • 生产者可以acquire这个lock,因为它已经被消费者release。
  • 当调用了condition的notify()方法后,消费者被唤醒,但唤醒不意味着它可以开始运行。
  • notify()并不释放lock,调用notify()后,lock依然被生产者所持有。
  • 生产者通过condition.release()显式释放lock。
  • 消费者再次开始运行,现在它可以得到队列中的数据而不会出现IndexError异常。

为队列增加大小限制

生产者不能向一个满队列继续加入数据。

它可以用以下方式来实现:

  • 在加入数据前,生产者检查队列是否为满。
  • 如果不为满,生产者可以继续正常流程。
  • 如果为满,生产者必须等待,调用condition实例的wait()。
  • 消费者可以运行。消费者消耗队列,并产生一个空余位置。
  • 然后消费者notify生产者。
  • 当消费者释放lock,消费者可以acquire这个lock然后往队列中加入数据。
最终版本如下:
from threading import Thread, Condition
import time
import random
 
queue = []
MAX_NUM = 10
condition = Condition()
 
class ProducerThread(Thread):
    def run(self):
        nums = range(5)
        global queue
        while True:
            condition.acquire()
            if len(queue) == MAX_NUM:
                print "Queue full, producer is waiting"
                condition.wait()
                print "Space in queue, Consumer notified the producer"
            num = random.choice(nums)
            queue.append(num)
            print "Produced", num
            condition.notify()
            condition.release()
            time.sleep(random.random())
 
class ConsumerThread(Thread):
    def run(self):
        global queue
        while True:
            condition.acquire()
            if not queue:
                print "Nothing in queue, consumer is waiting"
                condition.wait()
                print "Producer added something to queue and notified the consumer"
            num = queue.pop(0)
            print "Consumed", num
            condition.notify()
            condition.release()
            time.sleep(random.random())
 
ProducerThread().start()
ConsumerThread().start()
输出结果:
Produced 0
Consumed 0
Produced 0
Produced 4
Consumed 0
Consumed 4
Nothing in queue, consumer is waiting
Produced 4
Producer added something to queue and notified the consumer
Consumed 4
Produced 3
Produced 2
Consumed 3
Queue封装了Condition的行为,如wait(),notify(),acquire()。

利用Queue可以很方便的实现以上功能:

from threading import Thread
import time
import random
from Queue import Queue
 
queue = Queue(10)
 
class ProducerThread(Thread):
    def run(self):
        nums = range(5)
        global queue
        while True:
            num = random.choice(nums)
            queue.put(num)
            print "Produced", num
            time.sleep(random.random())
 
class ConsumerThread(Thread):
    def run(self):
        global queue
        while True:
            num = queue.get()
            queue.task_done()
            print "Consumed", num
            time.sleep(random.random())
 
ProducerThread().start()
ConsumerThread().start()

解释:

  • 在原来使用list的位置,改为使用Queue实例(下称队列)。
  • 这个队列有一个condition,它有自己的lock。如果你使用Queue,你不需要为condition和lock而烦恼。
  • 生产者调用队列的put方法来插入数据。
  • put()在插入数据前有一个获取lock的逻辑。
  • 同时,put()也会检查队列是否已满。如果已满,它会在内部调用wait(),生产者开始等待。
  • 消费者使用get方法。
  • get()从队列中移出数据前会获取lock。
  • get()会检查队列是否为空,如果为空,消费者进入等待状态。
  • get()和put()都有适当的notify()。现在就去看Queue的源码吧。

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