[ENH] informative failure message in `test_get_package_metadata_retur…

…ns_expected_results` (#239)

So far, `test_get_package_metadata_returns_expected_results` has failed
in every attempt to make a proper PR to `scikit-base`, due to the design
decision of using the package itself as a test case for `all_objects` -
so adding any function or class anywhere without making a corresponding
change in a register will fail the test.

Apart from that this needs to change, as a short term measure a more
helpful error message should allow developers to find the place to add
the expected functions or classes.

This PR adds these error messages.

skbase/lookup/tests/test_lookup.py

@@ -743,14 +743,43 @@ def test_get_package_metadata_returns_expected_results(
         if exclude_non_public_items:
             module_funcs = SKBASE_PUBLIC_FUNCTIONS_BY_MODULE.get(module, ())
             module_classes = SKBASE_PUBLIC_CLASSES_BY_MODULE.get(module, ())
+            which_str = "public"
+            fun_str = "SKBASE_PUBLIC_FUNCTIONS_BY_MODULE"
+            cls_str = "SKBASE_PUBLIC_CLASSES_BY_MODULE"
         else:
             module_funcs = SKBASE_FUNCTIONS_BY_MODULE.get(module, ())
             module_classes = SKBASE_CLASSES_BY_MODULE.get(module, ())
+            which_str = "all"
+            fun_str = "SKBASE_FUNCTIONS_BY_MODULE"
+            cls_str = "SKBASE_CLASSES_BY_MODULE"
 
         # Verify expected functions are returned
-        assert set(results[module]["functions"].keys()) == set(module_funcs)
+        retrieved_funcs = set(results[module]["functions"].keys())
+        expected_funcs = set(module_funcs)
+
+        if retrieved_funcs != expected_funcs:
+            msg = (
+                "When using get_package_metadata utility, retrieved objects "
+                f"for {which_str} functions in module {module} do not match expected. "
+                f"Expected: {expected_funcs}; "
+                f"retrieved: {retrieved_funcs}. "
+                f"Expected functions are stored in {fun_str}, in test_lookup."
+            )
+            raise AssertionError(msg)
+
         # Verify expected classes are returned
-        assert set(results[module]["classes"].keys()) == set(module_classes)
+        retrieved_cls = set(results[module]["classes"].keys())
+        expected_cls = set(module_classes)
+
+        if retrieved_cls != expected_cls:
+            msg = (
+                "When using get_package_metadata utility, retrieved objects "
+                f"for {which_str} classes in module {module} do not match expected. "
+                f"Expected: {expected_cls}; "
+                f"retrieved: {retrieved_cls}. "
+                f"Expected functions are stored in {cls_str}, in test_lookup."
+            )
+            raise AssertionError(msg)
 
         # Verify class metadata attributes correct
         for klass, klass_metadata in results[module]["classes"].items():