please critique my thread code

I wrote a Python program (103 lines, below) to download developer data
from SourceForge for research about social networks.

Please critique the code and let me know how to improve it.

An example use of the program:

promptpython download.py 1 240000

The above command downloads data for the projects with IDs between 1
and 240000, inclusive. As it runs, it prints status messages, with a
plus sign meaning that the project ID exists. Else, it prints a minus
sign.

Questions:

--- Are my setup and use of threads, the queue, and "while True" loop
correct or conventional?

--- Should the program sleep sometimes, to be nice to the SourceForge
servers, and so they don't think this is a denial-of-service attack?

--- Someone told me that popen is not thread-safe, and to use
mechanize. I installed it and followed an example on the web site.
There wasn't a good description of it on the web site, or I didn't
find it. Could someone explain what mechanize does?

--- How do I choose the number of threads? I am using a MacBook Pro
2.4GHz Intel Core 2 Duo with 4 GB 667 MHz DDR2 SDRAM, running OS
10.5.3.

Thank you.

Winston

#!/usr/bin/env python

# Winston C. Yang
# Created 2008-06-14

from __future__ import with_statement

import mechanize
import os
import Queue
import re
import sys
import threading
import time

lock = threading.RLock ()

# Make the dot match even a newline.
error_pattern = re.compile(".*\ n<!--pageid login -->\n.*", re.DOTALL)

def now():
return time.strftime(" %Y-%m-%d %H:%M:%S")

def worker():

while True:

try:
id = queue.get()
except Queue.Empty:
continue

request = mechanize.Reque st("http://sourceforge.net/project/"\
"memberlist.php ?group_id=%d" %
id)
response = mechanize.urlop en(request)
text = response.read()

valid_id = not error_pattern.m atch(text)

if valid_id:
f = open("%d.csv" % id, "w+")
f.write(text)
f.close()

with lock:
print "\t".join((str( id), now(), "+" if valid_id else
"-"))

def fatal_error():
print "usage: python application start_id end_id"
print
print "Get the usernames associated with each SourceForge project
with"
print "ID between start_id and end_id, inclusive."
print
print "start_id and end_id must be positive integers and satisfy"
print "start_id <= end_id."
sys.exit(1)

if __name__ == "__main__":

if len(sys.argv) == 3:

try:
start_id = int(sys.argv[1])

if start_id <= 0:
raise Exception

end_id = int(sys.argv[2])

if end_id < start_id:
raise Exception
except:
fatal_error()
else:
fatal_error()

# Print the start time.
start_time = now()
print start_time

# Create a directory whose name contains the start time.
dir = start_time.repl ace(" ", "_").replace(": ", "_")
os.mkdir(dir)
os.chdir(dir)

queue = Queue.Queue(0)

for i in xrange(32):
t = threading.Threa d(target=worker , name="worker %d" % (i +
1))
t.setDaemon(Tru e)
t.start()

for id in xrange(start_id , end_id + 1):
queue.put(id)

# When the queue has size zero, exit in three seconds.
while True:
if queue.qsize() == 0:
time.sleep(3)
break

print now()