docs: def get_validator(self, schema=None):
          """Get a jsonschema validator.
  
          :param dict schema: A custom schema to validate against.
          :returns: an instance of jsonschema Draft4Validator.
          """
          schema = schema if schema is not None else self.schema
          return jsonschema.Draft4Validator(
              schema, resolver=self.resolver,
              format_checker=jsonschema.draft4_format_checker)