is there any bug in this multi-thread script?

[Ju Hui]

I wrote a script to do work with multi-thread in a queue reference
others code.
Is there any bug in this script?
another question is : when we call start to run one thread extends from
threading, it will call a join() by itself? I want to monitor the
qsize() to ensure all work are finished. but when I commented the
#############code start
while q.qsize()>0:
    time.sleep(0.1)
#############code end
it works well too.
any response is welcome.
Thanks a lot.

#############code start

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

q=Queue.Queue(0)
NUM_WORKERS = 3

class MyThread(threading.Thread):
    """A worker thread."""
    def __init__(self, input, worktype):
        self._jobq = input
        self._work_type = worktype
        threading.Thread.__init__(self)
    def run(self):
        """
        Get a job and process it. Stop when there's no more jobs
        """
        while True:
            if self._jobq.qsize()>0:
                job = self._jobq.get()
                worktype=self._work_type
                self._process_job(job,worktype)
            else:
                break
    def _process_job(self, job,worktype):
        """
        Do useful work here.
        worktype: let this thread do different work
        1,do list
        2,do item
        3,,,
        """
        doJob(job)

def doJob(job):
    """
    do work function 1
    """
    time.sleep(random.random()*3)
    print "doing ",job
if __name__=='__main__':

    print "begin..."
    #put some work to q
    for i in range(NUM_WORKERS*2):
        q.put(i)
    #print total job q's size
    print "job q'size",q.qsize()
    #start threads to work
    for x in range(NUM_WORKERS):
        MyThread(q,x).start()
    #if q is not empty, wait
    #while q.qsize()>0:
    #    time.sleep(0.1)