Converting Pose-stamped from one frame to another

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Dear ROSers,

ROS_DISTRO: Melodic
OS: Ubuntu 18.04

I'm trying to, using tf instead of tf2 (which I do not know yet) convert a PoseStamped point from the world frame to a moving child frame test_tf. The point is given in the RViz window with the Navgoal button, which publishes a PoseStamped point to the relevant topic, in terms of the world frame. I get the point through a subscriber but I try to convert it with listener.transformPose, and it works at first, but I need to know it's relative position all the time, which does not get updated. I output the X position relative to world and to test_tf and check it in rqt_console but it does not update itself as the frame moves.

this is the Broadcaster (which makes the frame move in a sinusodial fashion):

def tf_pose(tf_name):
now = rospy.Time.now()
br.sendTransform((sin(now.secs), 0, 0),
                tf.transformations.quaternion_from_euler(0, 0, 0),
                now,
                tf_name,
                "world")

This is the listener

def handle_goal(goal):
    global navpoint
    rospy.loginfo("goal.x {}".format(goal.pose.position.x))
    navpoint = goal

if __name__ == '__main__':
    rospy.init_node('test_tf_listener')
    listener = tf.TransformListener()
    rospy.Subscriber('move_base_simple/goal',
                     PoseStamped,
                     handle_goal)
    navpoint = None
    rate = rospy.Rate(10.0)

    listener.waitForTransform("/world", "/test_tf", rospy.Time(), rospy.Duration(4.0))

    while not rospy.is_shutdown():
        try:
            (trans,rot) = listener.lookupTransform('/world', '/test_tf', rospy.Time(0))
            if navpoint is None:
                rospy.loginfo('navpoint is None')
            else:
                navpoint_loc = listener.transformPose('test_tf', navpoint)
                rospy.loginfo('navpoint.x: {} -> navpoint_l.x: {}'.format(navpoint.pose.position.x,navpoint_loc.pose.position.x))

        except (tf.LookupException, tf.ConnectivityException, tf.ExtrapolationException):
            continue

        rate.sleep()

The output I get from the rqt_console is:

navpoint.x: 0.0976972579956 -> navpoint_l.x: -0.889827448254
navpoint.x: 0.0976972579956 -> navpoint_l.x: -0.889827448254
navpoint.x: 0.0976972579956 -> navpoint_l.x: -0.889827448254

But I expect that the navpoint_l which is the point location relative to the test_tf should change as the frame moves.

I would like to know why it doesn't, and how can I access the most current available position of the point relative to the moving test_tf