def locked_get(self):
        """Retrieve Credential from file.

        Returns:
            oauth2client.client.Credentials
        """
        credentials = None
        content = keyring.get_password(self._service_name, self._user_name)

        if content is not None:
            try:
                credentials = client.Credentials.new_from_json(content)
                credentials.set_store(self)
            except ValueError:
                pass

        return credentials