fix: XmlPart._rel_ref_count

`.rel_ref_count()` as implemented was only applicable to `XmlPart` where
references to a related part could be present in the XML. Longer term it
probably makes sense to override `Part.drop_rel()` in `XmlPart` and not
have a `_rel_ref_count()` method in `part` at all, but this works and is
less potentially disruptive.

src/docx/opc/part.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, Callable, Dict, Type
+from typing import TYPE_CHECKING, Callable, Dict, Type, cast
 
 from docx.opc.oxml import serialize_part_xml
 from docx.opc.packuri import PackURI
@@ -149,11 +149,12 @@ def target_ref(self, rId: str) -> str:
         rel = self.rels[rId]
         return rel.target_ref
 
-    def _rel_ref_count(self, rId):
-        """Return the count of references in this part's XML to the relationship
-        identified by `rId`."""
-        rIds = self._element.xpath("//@r:id")
-        return len([_rId for _rId in rIds if _rId == rId])
+    def _rel_ref_count(self, rId: str) -> int:
+        """Return the count of references in this part to the relationship identified by `rId`.
+
+        Only an XML part can contain references, so this is 0 for `Part`.
+        """
+        return 0
 
 
 class PartFactory:
@@ -231,3 +232,9 @@ def part(self):
         That chain of delegation ends here for child objects.
         """
         return self
+
+    def _rel_ref_count(self, rId: str) -> int:
+        """Return the count of references in this part's XML to the relationship
+        identified by `rId`."""
+        rIds = cast("list[str]", self._element.xpath("//@r:id"))
+        return len([_rId for _rId in rIds if _rId == rId])

tests/opc/test_part.py

@@ -169,24 +169,13 @@ def it_can_establish_an_external_relationship(self, rels_prop_: Mock, rels_: Moc
         rels_.get_or_add_ext_rel.assert_called_once_with("http://rel/type", "https://hyper/link")
         assert rId == "rId27"
 
-    @pytest.mark.parametrize(
-        ("part_cxml", "rel_should_be_dropped"),
-        [
-            ("w:p", True),
-            ("w:p/r:a{r:id=rId42}", True),
-            ("w:p/r:a{r:id=rId42}/r:b{r:id=rId42}", False),
-        ],
-    )
-    def it_can_drop_a_relationship(
-        self, part_cxml: str, rel_should_be_dropped: bool, rels_prop_: Mock
-    ):
+    def it_can_drop_a_relationship(self, rels_prop_: Mock):
         rels_prop_.return_value = {"rId42": None}
-        part = Part("partname", "content_type")
-        part._element = element(part_cxml)  # pyright: ignore[reportAttributeAccessIssue]
+        part = Part(PackURI("/partname"), "content_type")
 
         part.drop_rel("rId42")
 
-        assert ("rId42" not in part.rels) is rel_should_be_dropped
+        assert "rId42" not in part.rels
 
     def it_can_find_a_related_part_by_reltype(
         self, rels_prop_: Mock, rels_: Mock, other_part_: Mock
@@ -411,6 +400,24 @@ def it_knows_its_the_part_for_its_child_objects(self, part_fixture):
         xml_part = part_fixture
         assert xml_part.part is xml_part
 
+    @pytest.mark.parametrize(
+        ("part_cxml", "rel_should_be_dropped"),
+        [
+            ("w:p", True),
+            ("w:p/r:a{r:id=rId42}", True),
+            ("w:p/r:a{r:id=rId42}/r:b{r:id=rId42}", False),
+        ],
+    )
+    def it_only_drops_a_relationship_with_zero_reference_count(
+        self, part_cxml: str, rel_should_be_dropped: bool, rels_prop_: Mock, package_: Mock
+    ):
+        rels_prop_.return_value = {"rId42": None}
+        part = XmlPart(PackURI("/partname"), "content_type", element(part_cxml), package_)
+
+        part.drop_rel("rId42")
+
+        assert ("rId42" not in part.rels) is rel_should_be_dropped
+
     # fixtures -------------------------------------------------------
 
     @pytest.fixture
@@ -452,6 +459,10 @@ def parse_xml_(self, request, element_):
     def partname_(self, request):
         return instance_mock(request, PackURI)
 
+    @pytest.fixture
+    def rels_prop_(self, request: FixtureRequest):
+        return property_mock(request, XmlPart, "rels")
+
     @pytest.fixture
     def serialize_part_xml_(self, request):
         return function_mock(request, "docx.opc.part.serialize_part_xml")