Make universe required in DAGMCCell.from_xml_element, remove stub

universe is now a required parameter since from_xml_element is always
called from a DAGMCUniverse context. The test uses a clearly named
placeholder_univ with a comment explaining the direct call is only
for testing purposes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: pshriwise

openmc/dagmc.py

@@ -636,7 +636,7 @@ def create_xml_subelement(self, xml_element, memo=None):
         return super().create_xml_subelement(xml_element, memo)
 
     @classmethod
-    def from_xml_element(cls, elem, mats, universe=None):
+    def from_xml_element(cls, elem, mats, universe):
         """Generate a DAGMCCell from an XML <cell> override element.
 
         Parameters
@@ -646,9 +646,8 @@ def from_xml_element(cls, elem, mats, universe=None):
         mats : dict
             Dictionary mapping material ID strings to
             :class:`openmc.Material` instances
-        universe : DAGMCUniverse, optional
-            Universe to add the parsed cell to. If not provided the cell is
-            returned without being added to any universe.
+        universe : DAGMCUniverse
+            Universe to add the parsed cell to.
 
         Returns
         -------
@@ -671,17 +670,6 @@ def from_xml_element(cls, elem, mats, universe=None):
             raise ValueError(
                 f"DAGMC cell {cell_id} must specify a material override.")
 
-        # Delegate to Cell.from_xml_element. Pass the DAGMCUniverse directly
-        # so the cell is registered there; if no universe is provided use a
-        # no-op stub so the cell is returned without being added anywhere.
-        if universe is not None:
-            target = universe
-        else:
-            class _Stub:
-                def add_cell(self, _):
-                    pass
-            target = _Stub()
-
         return super().from_xml_element(
             elem, surfaces={}, materials=mats,
-            get_universe=lambda _: target)
+            get_universe=lambda _: universe)

tests/unit_tests/test_cell.py

@@ -380,10 +380,14 @@ def test_dagmccell_from_xml_element():
     mat = openmc.Material(1)
     mat.add_nuclide('U235', 1.0)
     mats = {'1': mat}
+    # In practice, from_xml_element is always called from DAGMCUniverse
+    # during XML parsing. A placeholder universe is used here so the test
+    # can exercise the method directly without a real DAGMC model file.
+    placeholder_univ = openmc.DAGMCUniverse('model.h5m')
 
     # material + temperature + density round-trip
     xml = '<cell id="5" name="fuel" material="1" temperature="900.0" density="10.5"/>'
-    cell = openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats)
+    cell = openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats, placeholder_univ)
     assert cell.id == 5
     assert cell.name == 'fuel'
     assert cell.fill is mat
@@ -392,25 +396,25 @@ def test_dagmccell_from_xml_element():
 
     # volume round-trip
     xml = '<cell id="6" material="1" volume="42.0"/>'
-    cell = openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats)
+    cell = openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats, placeholder_univ)
     assert cell.volume == 42.0
 
     # forbidden: region, fill, universe
     for tag, val in [('region', '-1'), ('fill', '2'), ('universe', '0')]:
         xml = f'<cell id="7" material="1" {tag}="{val}"/>'
         with pytest.raises(ValueError, match=tag):
-            openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats)
+            openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats, placeholder_univ)
 
     # forbidden: translation, rotation
     for tag, val in [('translation', '1 0 0'), ('rotation', '0 0 90')]:
         xml = f'<cell id="7" material="1" {tag}="{val}"/>'
         with pytest.raises(ValueError, match=tag):
-            openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats)
+            openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats, placeholder_univ)
 
     # missing material raises
     xml = '<cell id="8"/>'
     with pytest.raises(ValueError, match='material'):
-        openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats)
+        openmc.DAGMCCell.from_xml_element(ET.fromstring(xml), mats, placeholder_univ)
 
 
 def test_plot(run_in_tmpdir):