import httplib

  def VerifyCredentials(self):

    '''Returns a twitter.User instance if the authenticating user is valid.

    Returns: 

      A twitter.User instance representing that user if the

      credentials are valid, None otherwise.

    '''

    if not self._username:

      raise TwitterError("Api instance must first be given user credentials.")

    url = 'http://twitter.com/account/verify_credentials.json'

    try:

      json = self._FetchUrl(url, no_cache=True)

    except urllib2.HTTPError, http_error:

      if http_error.code == httplib.UNAUTHORIZED:

        return None

      else:

        raise http_error

    data = simplejson.loads(json)

    self._CheckForTwitterError(data)

    return NewUserFromJsonDict(data)