title: My python script does not publish velocity command to parrot drone in ROS
body: <p>I would like to publish velocities for my <strong>ARDrone</strong> using <code>/cmd_vel</code> topic using the below python script. But it does nothing. It does not publish the required information.
  What is wrong in the below code?</p>
  <pre class="lang-py prettyprint-override"><code>#!/usr/bin/env python3
  
  import numpy as np
  import rospy
  from geometry_msgs.msg import Twist
  
  class KeyboardControl:
      def __init__(self):
          rospy.init_node('Script_controlling_ARDrone', anonymous=False)
          publisher = rospy.Publisher('/cmd_vel', Twist, queue_size=1)
  
          twist = Twist()
          twist.linear.x = 0
          twist.linear.y = 0
          twist.linear.z = -1
          twist.angular.x = 0
          twist.angular.y = 0
          twist.angular.z = 0.5
  
          publisher.publish(twist)
  
  
  def main():
      try:
          kc = KeyboardControl()
          rospy.spin()
      except KeyboardInterrupt:
          rospy.loginfo(&quot;Shutting down&quot;)
  
  if __name__ == '__main__':
      main()
  </code></pre>