|PackURI| instance containing partname targeted by this relationship.
        Raises ``ValueError`` on reference if target_mode is ``'External'``.
        Use :attr:`target_mode` to check before referencing.

def target_partname(self):
        """
        |PackURI| instance containing partname targeted by this relationship.
        Raises ``ValueError`` on reference if target_mode is ``'External'``.
        Use :attr:`target_mode` to check before referencing.
        """
        if self.is_external:
            msg = ('target_partname attribute on Relationship is undefined w'
                   'here TargetMode == "External"')
            raise ValueError(msg)
        # lazy-load _target_partname attribute
        if not hasattr(self, '_target_partname'):
            self._target_partname = PackURI.from_rel_ref(self._baseURI,
                                                         self.target_ref)
        return self._target_partname