my threaded program hangs after calling Queue.join()

[Babu]

I am not an expert in python, so any help is my appreciated.  My
threaded program consumed everything in the queue but is not
exiting... What is the problem with this program?

#!/opt/gspython-2.5/bin/python

""" Intention of this program: given a list of host names in a file,
get the .rhosts file, remove the newline and create a single record or
tuple
for each server.
"""

import paramiko
import os
import sys
import Queue
import threading
import pdb

SPath = "/.rhosts"  		#location of .rhosts in Solaris
LPath = "/root/.rhosts" #location of .rhosts in Linux

#Globals starts with Capital letter
Q_in	= Queue.Queue()

class	ProcessThreads(threading.Thread):
	""" Threaded remote execution and data gathering """
	def __init__(self, Q_in):
		threading.Thread.__init__(self)
		self.Q_in = Q_in


	def run(self):
		while True:
			#grabs host and path from Q_in
			host, path = self.Q_in.get()


			#print "host = ", host, "file = ", path
			if host is None:
				break # reached end of queue

			ssh_c = paramiko.SSHClient()
			ssh_c.load_system_host_keys()
			user = 'root'
			port = 22
			#remotely execute the commands
			try:
				ssh_c.connect(host, port, user, timeout=20)
			except Exception, e:
				print "ssh exception %s for %s" %( e, host)
				break


			stdin, stdout, stderr = ssh_c.exec_command('cat ' + path)
			output = stdout.readlines()
			outputs = ''.join(output).replace('\n',',')
			print host,": ", outputs

			#signals to queue job is done
			self.Q_in.task_done()


def usage():
	nargs = len(sys.argv)
	if  nargs < 2:
		print "USAGE: %s <input-file>"  % os.path.basename(sys.argv[0])
		print "<input-file> contains records of hostnames, one host per
line"
		sys.exit(1)

def load_ssh():
	paramiko.util.log_to_file('/var/tmp/paramiko.log')


if __name__ == "__main__":
	#pdb.set_trace()
	usage()
	hostfile = sys.argv[1]
	fh = open(hostfile, 'r')
	records = fh.readlines()
	fh.close()
	load_ssh()

	#spawn a pool of thread and spawn them queue instance
	for i in range(5):
		t = ProcessThreads(Q_in)
		t.setDaemon(True)
		t.start()

	#populate queue with data

	for recs in records:
		hostinfo = recs.strip()
		host, os = hostinfo.split()
		if os == "Linux":
			path = LPath
		else:
			path = SPath
		Q_in.put([host, path])

	for i in range(5):
		Q_in.put([None, None])  # end of queue signal

	Q_in.join()