def build(
        self, values: t.Mapping[str, t.Any], append_unknown: bool = True
    ) -> tuple[str, str] | None:
        """Assembles the relative url for that rule and the subdomain.
        If building doesn't work for some reasons `None` is returned.

        :internal:
        """
        try:
            if append_unknown:
                return self._build_unknown(**values)
            else:
                return self._build(**values)
        except ValidationError:
            return None
Assembles the relative url for that rule and the subdomain.
        If building doesn't work for some reasons `None` is returned.

        :internal:
        
src/werkzeug/routing/rules.py