title: Writing a ros node with both a publisher and subscriber?
body: <p>I am currently trying to make a ROS node in Python which has both a subscriber and a publisher.</p>
  
  <p>I've seen examples where a message is published within the callback, but I want it to "constantly" publish messages, and perform callbacks when it is the case. </p>
  
  <p>Here is how I do it now: </p>
  
  <pre><code>#!/usr/bin/env python
  
  import rospy
  from std_msgs.msg import Empty
  from std_msgs.msg import String
  import numpy as np
  
  pub = rospy.Publisher('/status', String, queue_size=1000)
  
  def callback(data):
      print "Message received"
  
  def listener():
  
      rospy.init_node('control', anonymous=True)
  
      rospy.Subscriber('control_c', Empty, callback)
      rospy.spin()
  
  if __name__ == '__main__':
      print "Running"
      listener()
  </code></pre>
  
  <p>So where should I publish?</p>