How to get the data from Subscribers message with python

edit

Hello, In my program, I try to get the Pose msg data which publish from other topic.

But when I print the variable which to read the Subscribers message, I will get the all values are 0.

Here is my code:

import sys
import rospy 
import geometry_msgs.msg 
from geometry_msgs.msg import Pose 

grasp_goal = Pose()

def callback(msg):
    global grasp_goal
    grasp_goal = msg.grasp_goal

# Create a ROS node.
rospy.init_node('move_to_grasp', anonymous=True)
rospy.loginfo('Start to grasp the object...')

# Subscribe to the ROS topic that contains the grasp data.
sub = rospy.Subscriber('/grasp_pose', Pose, callback)

rospy.loginfo("x:%f y:%f z:%f w:%f x:%f y:%f z:%f", grasp_goal.position.x, grasp_goal.position.y, grasp_goal.position.z, grasp_goal.orientation.w, grasp_goal.orientation.x, grasp_goal.orientation.y, grasp_goal.orientation.z)

This is echo /grasp_pose topic:

This is result, all value are zero:

Thanks for your help !