'Retrieve Credential from file.
Returns:
oauth2client.client.Credentials'

def locked_get(self):

    credentials = None
    content = keyring.get_password(self._service_name, self._user_name)
    if (content is not None):
        try:
            credentials = Credentials.new_from_json(content)
            credentials.set_store(self)
        except ValueError:
            pass
    return credentials
