code: def by_identifier(self, request):
          
          client_id = request.get_param("client_id")
  
          if client_id is None:
              raise OAuthInvalidNoRedirectError(error="missing_client_id")
  
          try:
              client = self.client_store.fetch_by_client_id(client_id)
          except ClientNotFoundError:
              raise OAuthInvalidNoRedirectError(error="unknown_client")
  
          redirect_uri = request.get_param("redirect_uri")
          if redirect_uri is not None:
              try:
                  client.redirect_uri = redirect_uri
              except RedirectUriUnknown:
                  raise OAuthInvalidNoRedirectError(
                      error="invalid_redirect_uri")
  
          return client
docs: Authenticates a client by its identifier.
  
          :param request: The incoming request
          :type request: oauth2.web.Request
  
          :return: The identified client
          :rtype: oauth2.datatype.Client
  
          :raises: :class OAuthInvalidNoRedirectError: