human
Complete this python program:
def _AddListFieldsMethod(message_descriptor, cls):
assistant
#Here is the Python code you requested.

    """Helper for _AddMessageMethods().
    """
    def ListFields(self):
        all_fields = [item for item in self._fields.iteritems() if _IsPresent(item)]
        all_fields.sort(key=(lambda item: item[0].number))
        return all_fields
    cls.ListFields = ListFields