code: def get_etags_and_matchers(self, request):
          
          # evaluate the preconditions, raises 428 if condition is not met
          self.evaluate_preconditions(request)
          # alright, headers are present, extract the values and match the conditions
          return super(APIETAGProcessor, self).get_etags_and_matchers(request)
docs: Get the etags from the header and perform a validation against the required preconditions.