code: def get_user(self, username=None, params=None):
          
          return self.transport.perform_request(
              "GET", _make_path("_security", "user", username), params=params
          )
docs: `<https://www.elastic.co/guide/en/elasticsearch/reference/current/security-api-get-user.html>`_
  
          :arg username: A comma-separated list of usernames