Test loading models with invalid file extensions. (#9955)

tests/python/test_model_io.py

@@ -254,6 +254,68 @@ class TestBoosterIO:
         # remove file
         Path.unlink(save_path)
 
+    def test_invalid_postfix(self) -> None:
+        """Test mis-specified model format, no special hanlding is expected, the
+        JSON/UBJ parser can emit parsing errors.
+
+        """
+        X, y, w = tm.make_regression(64, 16, False)
+        booster = xgb.train({}, xgb.QuantileDMatrix(X, y, weight=w), num_boost_round=3)
+
+        def rename(src: str, dst: str) -> None:
+            if os.path.exists(dst):
+                # Windows cannot overwrite an existing file.
+                os.remove(dst)
+            os.rename(src, dst)
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            path_dep = os.path.join(tmpdir, "model.deprecated")
+            # save into deprecated format
+            with pytest.warns(UserWarning, match="UBJSON"):
+                booster.save_model(path_dep)
+
+            path_ubj = os.path.join(tmpdir, "model.ubj")
+            rename(path_dep, path_ubj)
+
+            with pytest.raises(ValueError, match="{"):
+                xgb.Booster(model_file=path_ubj)
+
+            path_json = os.path.join(tmpdir, "model.json")
+            rename(path_ubj, path_json)
+
+            with pytest.raises(ValueError, match="{"):
+                xgb.Booster(model_file=path_json)
+
+            # save into ubj format
+            booster.save_model(path_ubj)
+            rename(path_ubj, path_dep)
+            # deprecated is not a recognized format internally, XGBoost can guess the
+            # right format
+            xgb.Booster(model_file=path_dep)
+            rename(path_dep, path_json)
+            with pytest.raises(ValueError, match="Expecting"):
+                xgb.Booster(model_file=path_json)
+
+            # save into JSON format
+            booster.save_model(path_json)
+            rename(path_json, path_dep)
+            # deprecated is not a recognized format internally, XGBoost can guess the
+            # right format
+            xgb.Booster(model_file=path_dep)
+            rename(path_dep, path_ubj)
+            with pytest.raises(ValueError, match="Expecting"):
+                xgb.Booster(model_file=path_ubj)
+
+            # save model without file extension
+            path_no = os.path.join(tmpdir, "model")
+            with pytest.warns(UserWarning, match="UBJSON"):
+                booster.save_model(path_no)
+
+            booster_1 = xgb.Booster(model_file=path_no)
+            r0 = booster.save_raw(raw_format="json")
+            r1 = booster_1.save_raw(raw_format="json")
+            assert r0 == r1
+
 
 def save_load_model(model_path: str) -> None:
     from sklearn.datasets import load_digits