listener.py

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#!/usr/bin/env python
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## Simple talker demo that listens to std_msgs/Strings published 
## to the 'chatter' topic

PKG = 'rospy_tutorials' # this package name
import roslib; roslib.load_manifest(PKG)

import rospy
from std_msgs.msg import String

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

    # in ROS, nodes are unique named. If two nodes with the same
    # node are launched, the previous one is kicked off. The 
    # anonymous=True flag means that rospy will choose a unique
    # name for our 'talker' node so that multiple talkers can
    # run simultaenously.
    rospy.init_node('listener', anonymous=True)

    rospy.Subscriber("chatter", String, callback)

    # spin() simply keeps python from exiting until this node is stopped
    rospy.spin()
        
if __name__ == '__main__':
    listener()