Test for an unexpected result from a wrapped metadata operation.

lib/iris/common/_split_attribute_dicts.py

@@ -117,6 +117,12 @@ def _inner_function(*args, **kwargs):
                 _convert_pairedkeys_dict_to_splitattrs(right),
             )
             result = result.__class__([left, right])
+        else:
+            message = (
+                f"Common-metadata operation {operation!r} returned a result which is "
+                f"neither a Mapping nor a pair of Mappings : {result!r}."
+            )
+            raise ValueError(message)
 
         return result
 

lib/iris/tests/unit/common/metadata/split_attribute_dicts/__init__.py

@@ -0,0 +1,6 @@
+# Copyright Iris contributors
+#
+# This file is part of Iris and is released under the LGPL license.
+# See COPYING and COPYING.LESSER in the root of the repository for full
+# licensing details.
+"""Unit tests for the :mod:`iris.common.metadata._split_attribute_dicts` package."""

lib/iris/tests/unit/common/metadata/split_attribute_dicts/test__adjust_for_split_attribute_dictionaries.py

@@ -0,0 +1,37 @@
+# Copyright Iris contributors
+#
+# This file is part of Iris and is released under the LGPL license.
+# See COPYING and COPYING.LESSER in the root of the repository for full
+# licensing details.
+"""
+Unit tests for the :class:`iris.common._split_attribute_dicts.adjust_for_split_attribute_dictionaries`.
+
+"""
+import pytest
+
+from iris.common._split_attribute_dicts import (
+    adjust_for_split_attribute_dictionaries,
+)
+
+
+class Test:
+    """
+    Test the decorator function + behaviour of decorated functions.
+
+    In this module, only exercise the return-value error check,
+    since **all other behaviour is tested by integration**.
+    See : :mod:`iris.tests.unit.common.metadata.test_CubeMetadata`.
+    """
+
+    def test_bad_return_value__fails(self):
+        @adjust_for_split_attribute_dictionaries
+        def sample_metadata_operation(*dictionary_args):
+            return [1, 2, 3]
+
+        message = (
+            "Common-metadata operation .*sample_metadata_operation .* "
+            "returned a result which is neither a Mapping nor a pair of Mappings "
+            r": \[1, 2, 3\]."
+        )
+        with pytest.raises(ValueError, match=message):
+            sample_metadata_operation({})