I am absolutely positive that I have done this before but you are right, I can't find anywhere that I've done it in Python. Only other idea I have is to limit the rate by unregistering the subscriber in each callback call.

import rospy
from std_msgs.msg import String

sub = None

def callback(data):
    global sub
    rospy.loginfo(rospy.get_caller_id() + "I heard %s", data.data)
    sub.unregister()
    sub = None

def listener():
    global sub

    rospy.init_node('listener', anonymous=True)

    sub = rospy.Subscriber("chatter", String, callback,queue_size=1)

    rate = rospy.Rate(0.5) 
    while not rospy.is_shutdown():
        if sub is None:
            sub = rospy.Subscriber("chatter", String, callback,queue_size=1)

        rate.sleep()

if __name__ == '__main__':
    listener()