Resolves a variable like :meth:`resolve` but returns the
        special `missing` value if it cannot be found.

def resolve_or_missing(self, key):
        """Resolves a variable like :meth:`resolve` but returns the
        special `missing` value if it cannot be found.
        """
        if self._legacy_resolve_mode:
            rv = self.resolve(key)
            if isinstance(rv, Undefined):
                rv = missing
            return rv
        return resolve_or_missing(self, key)