def __init__(self, message_listener, message_descriptor, key_checker):
self._message_listener = message_listener<EOL>self._message_descriptor = message_descriptor<EOL>self._key_checker = key_checker<EOL>self._values = {}<EOL>
Args:
  message_listener: A MessageListener implementation.
    The ScalarMap will call this object's Modified() method when it
    is modified.
  key_checker: A type_checkers.ValueChecker instance to run on keys
    inserted into this container.
  value_checker: A type_checkers.ValueChecker instance to run on values
    inserted into this container.