<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def refresh(self):
 """Refresh the OAuth token. Returns: string: The auth token. Raises: LoginException: If there was a problem refreshing the OAuth token. """

        res = gpsoauth.perform_oauth(
            self._email, self._master_token, self._android_id,
            service=self._scopes,
            app='com.google.android.keep',
            client_sig='38918a453d07199354f8b19af05ec6562ced5788'
        )
        if 'Auth' not in res:
            if 'Token' not in res:
                raise exception.LoginException(res.get('Error'))

        self._auth_token = res['Auth']
        return self._auth_token