code: def get_request_header(self):
          
          # resource is allowed to be null if it's not available yet (the Chrome
          # client does this for the first getentitybyid call)
          if self._client_id is not None:
              self._request_header.client_identifier.resource = self._client_id
          return self._request_header
docs: Return ``request_header`` for use when constructing requests.
  
          Returns:
              Populated request header.