In this chapter we'll create our first listener node, which will retrieve info from another publishing node.

Make sure your current path is the previously created package.

touch listener.py

sudo chmod +x listener.py

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

First we initialize the listener node and then tell it to subscribe to the previously created chatter node. Here we also need to mention what to do with any incoming data from the chatter node. In this case, this function is called callback.

Now anytime we get incoming data from the subscribed chatter node, the callback function will log the data.

Note here that we specify the type of the incoming data (String).

rosrun my_awesome_code talker
rosrun my_awesome_code listener

You should now be able to see the following output:

[INFO] [WallTime: 1439848277.141546] /listener_14364_1439848276913 \ I heard hello world 1439848277.14
[INFO] [WallTime: 1439848277.241519] /listener_14364_1439848276913 \ I heard hello world 1439848277.24
[INFO] [WallTime: 1439848277.341668] /listener_14364_1439848276913 \ I heard hello world 1439848277.34
[INFO] [WallTime: 1439848277.441579] /listener_14364_1439848276913 \ I heard hello world 1439848277.44