listener_once.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 and shuts down afterwards

from __future__ import print_function

import rospy
from std_msgs.msg import String


def callback(data):
    print(rospy.get_caller_id(), "I heard %s" % data.data)
    rospy.signal_shutdown("Received %s, exiting now"  % data.data)


def listener():
    rospy.init_node('listener', anonymous=True)
    rospy.sleep(rospy.get_param('delay', 0.0))
    rospy.Subscriber("chatter", String, callback)
    rospy.spin()


if __name__ == '__main__':
    listener()