Why isn't rospy.Rate.sleep() terminating?

edit

Hi, I have written a python script which takes two tf frames and writes the transform between them to a file. The abbreviated code of the script looks like this:

listener = tf.TransformListener()
# waiting for first transform between origin and pose
listener.waitForTransform(origin, pose, rospy.Time(), rospy.Duration(5.0))
rate = rospy.Rate(10)
while not rospy.is_shutdown():
    try:
         now = rospy.Time.now()
         listener.waitForTransform(origin, pose, now, rospy.Duration(1.0))
         (trans, rot) = listener.lookupTransform(origin, pose, now)
         ... write trans and rot to the file ...
    # this exception is usually thrown when waitForTransform timed out
    except tf.Exception as e: 
         break # get out of the loop
    rate.sleep()
... some last commands before the script terminates ...

Usually this script should terminate when no more tf frames are published because I leave the loop when waitForTransform(...) throws an exception. When I run this code and start playing a bagfile (use_sim_time true and --clock) the script does not terminate because it gets stuck at rate.sleep() when rosbag play is finished. Any ideas why it behaves this way?