How to use the twitterapi.DirectMessage function in TwitterAPI

since:
        Narrows the returned results to just those statuses created
        after the specified HTTP-formatted date. [optional]

    Returns:
      A sequence of twitterapi.DirectMessage instances
    '''
    url = 'http://twitter.com/direct_messages.json'
    if not self._username:
      raise TwitterError("The twitterapi.Api instance must be authenticated.")
    parameters = {}
    if since:
      parameters['since'] = since
    json = self._FetchUrl(url, parameters=parameters)
    data = simplejson.loads(json)
    return [DirectMessage.NewFromJsonDict(x) for x in data]

  @staticmethod
  def NewFromJsonDict(data):
    '''Create a new instance based on a JSON dict.

    Args:
      data: A JSON dict, as converted from the JSON in the twitter API
    Returns:
      A twitterapi.DirectMessage instance
    '''
    return DirectMessage(created_at=data.get('created_at', None),
                         recipient_id=data.get('recipient_id', None),
                         sender_id=data.get('sender_id', None),
                         text=data.get('text', None),
                         sender_screen_name=data.get('sender_screen_name', None),
                         id=data.get('id', None),
                         recipient_screen_name=data.get('recipient_screen_name', None))