writing the received data to a text file

Here is an example of opening a UDP socket in a Mininet host and writing the received packets to a file. h1 acts as a server and h2 acts as a client which is supposed to send a message ( for example "Hello world") to h1. h1 is suppose to receive this message and store the message and address in foo.txt file. But after implementing this code, although foo.txt is created but it's empty and does not contain any data or information. What's wrong?

mininetSocketTest.py:

#!/usr/bin/python

from mininet.topo import Topo, SingleSwitchTopo
from mininet.net import Mininet
from mininet.log import lg, info
from mininet.cli import CLI

def main():
    lg.setLogLevel('info')

    net = Mininet(SingleSwitchTopo(k=2))
    net.start()

    h1 = net.get('h1')
    p1 = h1.popen('python myServer.py -i %s &' % h1.IP())

    h2 = net.get('h2')
    h2.cmd('python myClient.py -i %s -m "hello world"' % h1.IP())

    CLI( net )
    p1.terminate()
    net.stop()

if __name__ == '__main__':
main()

myClient.py:

import socket, optparse

parser = optparse.OptionParser()
parser.add_option('-i', dest='ip', default='127.0.0.1')
parser.add_option('-p', dest='port', type='int', default=12345)
parser.add_option('-m', dest='msg')
(options, args) = parser.parse_args()

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.sendto(options.msg, (options.ip, options.port) )

myServer.py:

import socket, optparse

parser = optparse.OptionParser()
parser.add_option('-i', dest='ip', default='')
parser.add_option('-p', dest='port', type='int', default=12345)
(options, args) = parser.parse_args()

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.bind( (options.ip, options.port) )

f = open('foo.txt','w')
while True:
  data, addr = s.recvfrom(512)
  f.write("%s: %s\n" % (addr, data))
f.flush()