python多线程问题

Question

任何人都可以告诉我为什么这段代码在启动线程后生成队列吗？基本上，queue是在for循环之后生成的，但是在ThreadUrl类中它已经使用queue.get（）方法。这个怎么用？它如何从尚未生成的队列中获取值？

for i in range(5): 
    t = ThreadUrl(queue, out_queue) 
    t.setDaemon(True) 
    t.start() 

# This is what confuses me! Shouldn't it be above the for loop?? 
for host in hosts: 
    queue.put(host) 

for i in range(5): 
    dt = DatamineThread(out_queue) 
    dt.setDaemon(True) 
    dt.start() 

#wait on the queue until everything has been processed 
queue.join() 
out_queue.join() 

以下是完整的源

import Queue 
import threading 
import urllib2 
import time 
from BeautifulSoup import BeautifulSoup 

hosts = ["http://yahoo.com", "http://google.com", "http://amazon.com", 
     "http://ibm.com", "http://apple.com"] 

queue = Queue.Queue() 
out_queue = Queue.Queue() 

class ThreadUrl(threading.Thread): 
    """Threaded Url Grab""" 
    def __init__(self, queue, out_queue): 
     threading.Thread.__init__(self) 
     self.queue = queue 
     self.out_queue = out_queue 

    def run(self): 
     while True: 
      #grabs host from queue 
      host = self.queue.get() 

      #grabs urls of hosts and then grabs chunk of webpage 
      url = urllib2.urlopen(host) 
      chunk = url.read() 

      #place chunk into out queue 
      self.out_queue.put(chunk) 

      #signals to queue job is done 
      self.queue.task_done() 

class DatamineThread(threading.Thread): 
    """Threaded Url Grab""" 
    def __init__(self, out_queue): 
     threading.Thread.__init__(self) 
     self.out_queue = out_queue 

    def run(self): 
     while True: 
      #grabs host from queue 
      chunk = self.out_queue.get() 

      #parse the chunk 
      soup = BeautifulSoup(chunk) 
      print soup.findAll(['title']) 

      #signals to queue job is done 
      self.out_queue.task_done() 

start = time.time() 
def main(): 

    #spawn a pool of threads, and pass them queue instance 
    for i in range(5): 
     t = ThreadUrl(queue, out_queue) 
     t.setDaemon(True) 
     t.start() 

    #populate queue with data 
    for host in hosts: 
     queue.put(host) 

    for i in range(5): 
     dt = DatamineThread(out_queue) 
     dt.setDaemon(True) 
     dt.start() 


    #wait on the queue until everything has been processed 
    queue.join() 
    out_queue.join() 

main() 
print "Elapsed Time: %s" % (time.time() - start) 

Answer 1

线host = self.queue.get()块执行的线程，直到一些元件出现在queue。

所以

#spawn a pool of threads, and pass them queue instance 
for i in range(5): 
    t = ThreadUrl(queue, out_queue) 
    t.setDaemon(True) 
    t.start() 

创建5个线程正在等待队列中的任何元素。

#populate queue with data 
for host in hosts: 
    queue.put(host) 

填满队列。这些线程开始处理之后。