code: def target_partname(self):
          
          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
docs: |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.