Thread locking question.

[Piet van Oostrum]

>>>>> grocery_stocker <cdalten at gmail.com> (gs) wrote:

>gs> The following code gets data from 5 different websites at the "same
>gs> time".

>gs> #!/usr/bin/python

>gs> import Queue
>gs> import threading
>gs> import urllib2
>gs> import time

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

>gs> queue = Queue.Queue()

>gs> class MyUrl(threading.Thread):
>gs>     def __init__(self, queue):
>gs>         threading.Thread.__init__(self)
>gs>         self.queue = queue

>gs>     def run(self):
>gs>         while True:
>gs>             host = self.queue.get()
>gs>             if host is None:
>gs>                 break
>gs>             url = urllib2.urlopen(host)
>gs>             print url.read(1024)
>gs>             #self.queue.task_done()

>gs> start = time.time()

>gs> def main():
>gs>     for i in range(5):
>gs>         t = MyUrl(queue)
>gs>         t.setDaemon(True)
>gs>         t.start()

>gs>     for host in hosts:
>gs>         print "pushing", host
>gs>         queue.put(host)

>gs>     for i in range(5):
>gs>         queue.put(None)

>gs>     t.join()

>gs> if __name__ == "__main__":
>gs>     main()
>gs>     print "Elapsed Time: %s" % (time.time() - start)


>gs> How does the parallel download work if each thread has a lock? When
>gs> the program opens www.yahoo.com, it places a lock on the thread,
>gs> right? If so, then doesn't that mean the other 4 sites have to wait
>gs> for the thread to release the lock?

No. Where does it set a lock? There is only a short lock period in the queue
when an item is put in the queue or got from the queue. And of course we
have the GIL, but this is released as soon as a long during operation is
started - in this case when the Internet communication is done.
-- 
Piet van Oostrum <piet at cs.uu.nl>
URL: http://pietvanoostrum.com [PGP 8DAE142BE17999C4]
Private email: piet at vanoostrum.org