The for loop is launching a number of worker threads to perform the function defined by "worker". Here is working code that should run on your system in python 2.7.

import Queue
import threading

# input queue to be processed by many threads
q_in = Queue.Queue(maxsize=0)

# output queue to be processed by one thread
q_out = Queue.Queue(maxsize=0)

# number of worker threads to complete the processing
num_worker_threads = 10

# process that each worker thread will execute until the Queue is empty
def worker():
    while True:
        # get item from queue, do work on it, let queue know processing is done for one item
        item = q_in.get()
        q_out.put(do_work(item))
        q_in.task_done()

# squares a number and returns the number and its square
def do_work(item):
    return (item,item*item)

# another queued thread we will use to print output
def printer():
    while True:
        # get an item processed by worker threads and print the result. Let queue know item has been processed
        item = q_out.get()
        print "%d squared is : %d" % item
        q_out.task_done()

# launch all of our queued processes
def main():
    # Launches a number of worker threads to perform operations using the queue of inputs
    for i in range(num_worker_threads):
         t = threading.Thread(target=worker)
         t.daemon = True
         t.start()

    # launches a single "printer" thread to output the result (makes things neater)
    t = threading.Thread(target=printer)
    t.daemon = True
    t.start()

    # put items on the input queue (numbers to be squared)
    for item in range(10):
        q_in.put(item)

    # wait for two queues to be emptied (and workers to close)   
    q_in.join()       # block until all tasks are done
    q_out.join()

    print "Processing Complete"

main()

Python 3 version per @handle

import queue 
import threading

# input queue to be processed by many threads
q_in = queue.Queue(maxsize=0) 

# output queue to be processed by one thread
q_out = queue.Queue(maxsize=0) 

# number of worker threads to complete the processing
num_worker_threads = 10

# process that each worker thread will execute until the Queue is empty
def worker():
    while True:
        # get item from queue, do work on it, let queue know processing is done for one item
        item = q_in.get()
        q_out.put(do_work(item))
        q_in.task_done()

# squares a number and returns the number and its square
def do_work(item):
    return (item,item*item)

# another queued thread we will use to print output
def printer():
    while True:
        # get an item processed by worker threads and print the result. Let queue know item has been processed
        item = q_out.get()
        print("{0[0]} squared is : {0[1]}".format(item) )
        q_out.task_done()

# launch all of our queued processes
def main():
    # Launches a number of worker threads to perform operations using the queue of inputs
    for i in range(num_worker_threads):
         t = threading.Thread(target=worker)
         t.daemon = True
         t.start()

    # launches a single "printer" thread to output the result (makes things neater)
    t = threading.Thread(target=printer)
    t.daemon = True
    t.start()

    # put items on the input queue (numbers to be squared)
    for item in range(10):
        q_in.put(item)

    # wait for two queues to be emptied (and workers to close)   
    q_in.join()       # block until all tasks are done
    q_out.join()

    print( "Processing Complete" )

main()

A kinda standard cross py2-py3 compatible version:

try: 
    import queue
except ImportError:
    import Queue as queue

When you call queue.join() in the main thread, all it does is block the main threads until the workers have processed everything that's in the queue. It does not stop the worker threads, which continue executing their infinite loops.

If the worker threads are non-deamon, their continuing execution prevents the program from stopping irrespective of whether the main thread has finished.

Queue module is python 2 specific and queue module is python 3+

Importing the Queue module in python 3 will result in error

You do

from queue import *

This imports all the classes from the queue module already. Change that line to

q = Queue(maxsize=0)

CAREFUL: "Wildcard imports (from import *) should be avoided, as they make it unclear which names are present in the namespace, confusing both readers and many automated tools". (Python PEP-8)

As an alternative, one could use:

from queue import Queue

q = Queue(maxsize=0)

Yes. We can Optimize for the time cost of m function calls on your queue. This optimization can be any mix of enqueue and dequeue calls.

Assume you already have a stack implementation and it gives O(1)O(1) time push and pop.

#

class Stack():

    def __init__(self):
        self.stk = []

    def pop(self):
        """raises IndexError if you pop when it's empty"""
        return self.stk.pop()

    def push(self, elt):
        self.stk.append(elt)

    def is_empty(self):
        return len(self.stk) == 0

    def peek(self):
        if not self.stk.is_empty():
            return self.stk[-1]


class Queue():

    def __init__(self):
        self.q = Stack()  # the primary queue
        self.b = Stack()  # the reverse, opposite q (a joke: q vs b)
        self.front = None

    def is_empty(self):
        return self.q.is_empty()

    def peek(self):
        if self.q.is_empty():
            return None
        else:
            return self.front

    def enqueue(self, elt):
        self.front = elt
        self.q.push(elt)

    def dequeue(self):
        """raises IndexError if you dequeue from an empty queue"""
        while not self.q.is_empty() > 0:
            elt = self.q.pop()
            self.b.push(elt)
        val = self.b.pop()
        elt = None
        while not self.b.is_empty() > 0:
            elt = self.b.pop()
            self.q.push(elt)
        self.front = elt
        return val


# Now let's test


class TestQueueTwoStacks(unittest.TestCase):

    def setUp(self):
        self.q = Queue()

    def test_queuedequue(self):
        """queue up 5 integers, check they are in there, dequeue them, check for emptiness, perform other blackbox and whitebox tests"""
        self.assertTrue(self.q.is_empty())
        self.assertTrue(self.q.q.is_empty())
        self.assertTrue(self.q.b.is_empty())

        l = range(5)
        for i in l:
            self.q.enqueue(i)

        self.assertEqual(4, self.q.peek())
        self.assertEqual(l, self.q.q.stk)

        s = []
        l.reverse()
        for i in l:
            elt = self.q.dequeue()
            s.append(elt)

        self.assertTrue(self.q.is_empty())
        self.assertTrue(self.q.q.is_empty())
        self.assertTrue(self.q.b.is_empty())

        l.reverse()
        self.assertEqual(s, l)
        self.assertEqual([], self.q.b.stk)
        self.assertEqual([], self.q.q.stk)

if __name__ == "__main__":
    # unittest.main()
    suite = unittest.TestLoader().loadTestsFromTestCase(TestQueueTwoStacks)
    unittest.TextTestRunner(verbosity=2).run(suite)