Retrieve velocity data from twist messages

edit

hi, you probably want to do something like this (this is an example that converts geometry_msgs/Twist message to turtlesim/Velocity message):

def TwistToTurtle(): #initialization
rospy.init_node('TwistToTurtle',anonymous=False)
rospy.Subscriber('cmd_vel',Twist,gotTwistCB) #subscribe to cmd_vel topic which publishes Twist messages 
pub = rospy.Publisher('turtle1/command_velocity',Velocity)
rospy.spin()

def gotTwistCB(data): #when Twist msg is received
#take values from data and do your job, for example:
outdata = Velocity()
outdata.linear = data.linear.x
outdata.angular = data.angular.z
pub.publish(outdata)

SO data.linear.x/y/z and data.angular.x/y/z are the ones that you care about

more on subsribing tutorial here. Also, you may want to check out rosmsg or just run

rosmsg -h