python-openxml
diff --git a/‎opc/pkgreader.py‎
Lines changed: 19 additions & 1 deletion b/‎opc/pkgreader.py‎
Lines changed: 19 additions & 1 deletion
diff --git a/‎tests/test_pkgreader.py‎
Lines changed: 26 additions & 0 deletions b/‎tests/test_pkgreader.py‎
Lines changed: 26 additions & 0 deletions
@@ -14,7 +14,7 @@
 
 from opc.constants import RELATIONSHIP_TARGET_MODE as RTM
 from opc.oxml import oxml_fromstring
-from opc.packuri import PACKAGE_URI
+from opc.packuri import PACKAGE_URI, PackURI
 from opc.phys_pkg import PhysPkgReader
 
 
@@ -126,6 +126,7 @@ class _SerializedRelationship(object):
     """
     def __init__(self, baseURI, rel_elm):
         super(_SerializedRelationship, self).__init__()
+        self._baseURI = baseURI
         self._rId = rel_elm.rId
         self._reltype = rel_elm.reltype
         self._target_mode = rel_elm.target_mode
@@ -168,6 +169,23 @@ def target_ref(self):
         """
         return self._target_ref
 
+    @property
+    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
+
 
 class _SerializedRelationshipCollection(object):
     """
 
@@ -206,6 +206,32 @@ def it_knows_when_it_is_external(self):
             srel = _SerializedRelationship(None, rel_elm)
             assert srel.is_external is expected_value
 
+    def it_can_calculate_its_target_partname(self):
+        # test data --------------------
+        cases = (
+            ('/', 'docProps/core.xml', '/docProps/core.xml'),
+            ('/ppt', 'viewProps.xml', '/ppt/viewProps.xml'),
+            ('/ppt/slides', '../slideLayouts/slideLayout1.xml',
+             '/ppt/slideLayouts/slideLayout1.xml'),
+        )
+        for baseURI, target_ref, expected_partname in cases:
+            # setup --------------------
+            rel_elm = Mock(name='rel_elm', rId=None, reltype=None,
+                           target_ref=target_ref, target_mode=RTM.INTERNAL)
+            # exercise -----------------
+            srel = _SerializedRelationship(baseURI, rel_elm)
+            # verify -------------------
+            assert srel.target_partname == expected_partname
+
+    def it_raises_on_target_partname_when_external(self):
+        rel_elm = Mock(
+            name='rel_elm', rId='rId9', reltype='ReLtYpE',
+            target_ref='docProps/core.xml', target_mode=RTM.EXTERNAL
+        )
+        srel = _SerializedRelationship('/', rel_elm)
+        with pytest.raises(ValueError):
+            srel.target_partname
+
 
 class Describe_SerializedRelationshipCollection(object):