Give a more useful error message for entity merges

rock_paper_sand/wikidata.py

@@ -153,8 +153,15 @@ def entity(
                 f"https://www.wikidata.org/wiki/Special:EntityData/{entity_ref.id}.json"  # pylint: disable=line-too-long
             )
             response.raise_for_status()
+            entities = response.json()["entities"]
+            if entity_ref.id not in entities:
+                raise ValueError(
+                    f"JSON data for {entity_ref} does not contain "
+                    f"{entity_ref.id!r}. Maybe it was merged with another "
+                    "entity?"
+                )
             self._entity_by_ref[entity_ref] = wikidata_value.Entity(
-                json_full=response.json()["entities"][entity_ref.id],
+                json_full=entities[entity_ref.id],
             )
         return self._entity_by_ref[entity_ref]
 

rock_paper_sand/wikidata_test.py

@@ -129,6 +129,13 @@ def test_entity(self) -> None:
             self._mock_session.mock_calls,
         )
 
+    def test_entity_merged(self) -> None:
+        self._mock_session.get.return_value.json.return_value = {
+            "entities": {"Q2": {}}
+        }
+        with self.assertRaisesRegex(ValueError, "Q1.*merged"):
+            self._api.entity(wikidata_value.ItemRef("Q1"))
+
     def test_sparql(self) -> None:
         self._mock_session.get.return_value.json.return_value = {
             "results": {"bindings": [{"foo": "bar"}]}