def resolver(self):
        """jsonschema RefResolver object for the base schema."""
        if self._resolver is not None:
            return self._resolver
        if self._schema_path is not None:
            # the documentation for ref resolving
            # https://github.com/Julian/jsonschema/issues/98
            # https://python-jsonschema.readthedocs.org/en/latest/references/
            self._resolver = SchemaRefResolver(
                'file://' + self._schema_path + '/', self.schema)
        else:
            self._resolver = SchemaRefResolver.from_schema(self.schema)
        return self._resolver
jsonschema RefResolver object for the base schema.