Question

So, how does this code exit the while statement when the thread is started? (Please do not consider indentation)

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()

** EDIT *

The code that starts the thread:

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()

    queue.join()

Answer 1

It doesn’t have to exit the while statement for the code to terminate. All that is happening here is that the thread has consumed everything in the queue at which point queue.join() returns.

As soon as the call to queue.join() in the main code returns the main code will exit and because you marked the thread as a daemon the entire application will exit and your background thread will be killed.