Helper for :meth:`build`.  Returns subdomain and path for the
        rule that accepts this endpoint, values and method.

        :internal:

def _partial_build(self, endpoint, values, method, append_unknown):
        """Helper for :meth:`build`.  Returns subdomain and path for the
        rule that accepts this endpoint, values and method.

        :internal:
        """
        # in case the method is none, try with the default method first
        if method is None:
            rv = self._partial_build(
                endpoint, values, self.default_method, append_unknown
            )
            if rv is not None:
                return rv

        # default method did not match or a specific method is passed,
        # check all and go with first result.
        for rule in self.map._rules_by_endpoint.get(endpoint, ()):
            if rule.suitable_for(values, method):
                rv = rule.build(values, append_unknown)
                if rv is not None:
                    return rv