You should look into this tutorial: http://wiki.ros.org/ROS/Tutorials/Wri...

In the tutorial, the publisher and subscriber are in separate nodes, but you should be able to integrate the two.

EDIT: Here is an example:

#!/usr/bin/env python
# license removed for brevity
import rospy
from std_msgs.msg import String

def callback(data):
    rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.data)

def talker():
    pub = rospy.Publisher('chatter', String, queue_size=10)
    rate = rospy.Rate(10) # 10hz
    hello_str = "hello world %s" % rospy.get_time()
    rospy.loginfo(hello_str)
    pub.publish(hello_str)
    rate.sleep()

def listener():

    rospy.Subscriber("chatter", String, callback)
    rate = rospy.Rate(10)
    rate.sleep()

if __name__ == '__main__':
    rospy.init_node('talker_and_listener', anonymous=True)

    while not rospy.is_shutdown():
        talker()
        listener()