def get_conn(self):
        """
        Retrieves the connection to Cloud Firestore.

        :return: Google Cloud Firestore services object.
        """
        if not self._conn:
            http_authorized = self._authorize()
            # We cannot use an Authorized Client to retrieve discovery document due to an error in the API.
            # When the authorized customer will send a request to the address below
            # https://www.googleapis.com/discovery/v1/apis/firestore/v1/rest
            # then it will get the message below:
            # > Request contains an invalid argument.
            # At the same time, the Non-Authorized Client has no problems.
            non_authorized_conn = build("firestore", self.api_version, cache_discovery=False)
            self._conn = build_from_document(
                non_authorized_conn._rootDesc,  # pylint: disable=protected-access
                http=http_authorized
            )
        return self._conn 

def get_service(api='gmail', version='v1', auth='service', scopes=None, uri_file=None):
  global DISCOVERY_CACHE 

  key = api + version + auth + str(threading.current_thread().ident)

  if key not in DISCOVERY_CACHE:
    credentials = get_credentials(auth)
    if uri_file:
      uri_file = uri_file.strip()
      if uri_file.startswith('{'):
        DISCOVERY_CACHE[key] = discovery.build_from_document(uri_file, credentials=credentials)
      else:
        with open(uri_file, 'r') as cache_file:
          DISCOVERY_CACHE[key] = discovery.build_from_document(cache_file.read(), credentials=credentials)
    else:
      DISCOVERY_CACHE[key] = discovery.build(api, version, credentials=credentials)

  return DISCOVERY_CACHE[key] 

def _build_service_from_document(credentials, document_path):
    """Builds an API client from a local discovery document

    Args:
        credentials (OAuth2Credentials): Credentials that will be used to
            authenticate the API calls.
        document_path (str): The local path of the discovery document

    Returns:
        object: A Resource object with methods for interacting with the service.
    """
    with open(document_path, 'r') as f:
        discovery_data = json.load(f)

    return discovery.build_from_document(
        service=discovery_data,
        credentials=credentials
    )


# pylint: disable=too-many-instance-attributes 

def get_authenticated_service(args, credentials_folder):

    client_secrets_file = credentials_folder + "client_secrets.json"

    youtube_read_write_ssl_scope = "https://www.googleapis.com/auth/youtube.force-ssl"
    youtube_api_service_name = "youtube"
    youtube_api_version = "v3"

    missing_clients_secrets_message = """
    WARNING: Please configure OAuth 2.0

    To make this sample run you will need to populate the client_secrets.json file
    found at:
       %s
    with information from the APIs Console
    https://developers.google.com/console

    For more information about the client_secrets.json file format, please visit:
    https://developers.google.com/api-client-library/python/guide/aaa_client_secrets
    """ % os.path.abspath(os.path.join(os.path.dirname(__file__),
                                       client_secrets_file))

    flow = flow_from_clientsecrets(client_secrets_file,
                                   scope=youtube_read_write_ssl_scope,
                                   message=missing_clients_secrets_message)

    storage = Storage(credentials_folder + "reveal-stored-oauth2.json")
    credentials = storage.get()

    if credentials is None or credentials.invalid:
        credentials = run_flow(flow, storage, args)

    # Trusted testers can download this discovery document from the developers page
    #  and it should be in the same directory with the code.
    with open(credentials_folder + "youtube-v3-discoverydocument.json", "r") as f:
        doc = f.read()
        return build_from_document(doc, credentials=credentials)