How to set a model's position using /gazebo/set_model_state service in python

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The code below shows a node written in python, that will set the my_robot's position to the origin of the map. The service requires a message of type gazebo_msgs::ModelState.

import rospy 
import rospkg 
from gazebo_msgs.msg import ModelState 
from gazebo_msgs.srv import SetModelState

def main():
    rospy.init_node('set_pose')

    state_msg = ModelState()
    state_msg.model_name = 'my_robot'
    state_msg.pose.position.x = 0
    state_msg.pose.position.y = 0
    state_msg.pose.position.z = 0.3
    state_msg.pose.orientation.x = 0
    state_msg.pose.orientation.y = 0
    state_msg.pose.orientation.z = 0
    state_msg.pose.orientation.w = 0

    rospy.wait_for_service('/gazebo/set_model_state')
    try:
        set_state = rospy.ServiceProxy('/gazebo/set_model_state', SetModelState)
        resp = set_state( state_msg )

    except rospy.ServiceException, e:
        print "Service call failed: %s" % e

if __name__ == '__main__':
    try:
        main()
    except rospy.ROSInterruptException:
        pass