This method returns a GraphQLQuery (from cache if present)

def document_from_string(self, schema, request_string):
        # type: (GraphQLSchema, str) -> Optional[GraphQLDocument]
        """This method returns a GraphQLQuery (from cache if present)"""
        key = self.get_key_for_schema_and_document_string(schema, request_string)
        if key not in self.cache_map:
            self.cache_map[key] = self.backend.document_from_string(
                schema, request_string
            )

        return self.cache_map[key]