Access field in message inside a message

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Hallo

Being new to ros, I´ve stumbled into a stupid problem..

Im writing a python listener, and I want to get the orientation from an Imu.

The variable I want to access can be echoed with : rostopic echo /imu/orientation/z

Here is what I have so far - which is not really working out...:

#!/usr/bin/env python
import rospy
from sensor_msgs.msg import Imu
from geometry_msgs.msg import Quaternion
def imu_callback(data):
    rospy.loginfo(Imu.orientation.z)
   #do something with z

def imu_listener():
    rospy.init_node('listener', anonymous=True)
    rospy.Subscriber("imu", Imu, imu_callback)
    rospy.spin()
if __name__ == '__main__':
    imu_listener()