multithreading - Threading in python using queue

Question

I wanted to use threading in python to download lot of webpages and went through the following code which uses queues in one of the website.

it puts a infinite while loop. Does each of thread run continuously with out ending till all of them are complete? Am I missing something.

#!/usr/bin/env python
import Queue
import threading
import urllib2
import time

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

queue = Queue.Queue()

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

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

      #grabs urls of hosts and prints first 1024 bytes of page
      url = urllib2.urlopen(host)
      print url.read(1024)

      #signals to queue job is done
      self.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)
    t.setDaemon(True)
    t.start()

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

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

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

Answer 1

Setting the thread's to be daemon threads causes them to exit when the main is done. But, yes you are correct in that your threads will run continuously for as long as there is something in the queue else it will block.

The documentation explains this detail Queue docs

The python Threading documentation explains the daemon part as well.

The entire Python program exits when no alive non-daemon threads are left.

So, when the queue is emptied and the queue.join resumes when the interpreter exits the threads will then die.

EDIT: Correction on default behavior for Queue