Keys to twist slow.py

#!/usr/bin/env python
# Morgan Quigley, Brian Gerkey, & William D. Smart
# Programming Robots with ROS
# O'reilly 2015, p 114.
# BEGIN ALL
import rospy
from std_msgs.msg import String
from geometry_msgs.msg import Twist

# BEGIN KEYMAP
key_mapping = { 'w': [ 0, 1], 'x': [0, -1], 
                'a': [1, 0], 'd': [-1,  0], 
                's': [ 0, 0] }
# END KEYMAP

def keys_cb(msg, twist_pub):
  # BEGIN CB
  if len(msg.data) == 0 or not key_mapping.has_key(msg.data[0]):
    return # unknown key.
  vels = key_mapping[msg.data[0]]
  # END CB
  t = Twist()
  t.angular.z = vels[0]
  t.linear.x  = vels[1]
  twist_pub.publish(t)

if __name__ == '__main__':
  rospy.init_node('keys_to_twist_slow')
  twist_pub = rospy.Publisher('cmd_vel', Twist, queue_size=1)
  rospy.Subscriber('keys', String, keys_cb, twist_pub)
  rospy.spin()
# END ALL