def METHOD_NAME(self):
    with self._signing_credentials_lock:
        if self._signing_credentials is None or self._signing_credentials.expired:
            credentials, _ = google.auth.default(['https://www.googleapis.com/auth/cloud-platform'])
            auth_req = google.auth.transport.requests.Request()
            credentials.refresh(auth_req)
            self._signing_credentials = credentials
    return self._signing_credentials