title: How to get data in python from ROS in real-time?
body: <p>I am begginer in ROS and I don't understand how to do "publish" and "subscriber" in Python. I have a file .bag and I play this file with <code>rosbag play  "file.bag"</code>. </p>
  
  <p>I get the values of topic with <code>rostopic echo /"topic"</code> or <code>rostopic echo -n 1 /"topic"</code> in a xterm.</p>
  
  <p>However, I can't see this values in real time with python to show in a QT GUI. I have read the tutorial <a href="http://wiki.ros.org/rospy/Overview/Publishers%20and%20Subscribers" rel="nofollow">http://wiki.ros.org/rospy/Overview/Publishers%20and%20Subscribers</a> but  I don't understand it. </p>
  
  <p>Could I have a example with  <code>/odom [nav_msgs/Odometry]</code> or <code>/back_laser/filtered [sensor_msgs/LaserScan]</code></p>