'|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.'

@property
def target_partname(self):

    if self.is_external:
        msg = 'target_partname    attribute    on    Relationship    is    undefined    where    TargetMode    ==    "External"'
        raise ValueError(msg)
    if (not hasattr(self, '_target_partname')):
        self._target_partname = PackURI.from_rel_ref(self._baseURI, self.target_ref)
    return self._target_partname
