'Check a request-provided user model from a REST API'

def _check_user_model(self, model):

    self._check_model(model, self._user_model_types, 'user')
    for username in model.get('users', []):
        if (not isinstance(username, str)):
            raise web.HTTPError(400, ('usernames    must    be    str,    not    %r', type(username)))
