Raise with a message when reading a value that has no value and no reason

pipeline/fiftyone_pipeline_core/src/fiftyone_pipeline_core/aspectproperty_value.py

@@ -21,6 +21,11 @@
 # *********************************************************************
 
 
+# The message value() raises with when there is no value and no reason was
+# given, the same as .NET's.
+NO_VALUE_MESSAGE = "This instance does not have a set value"
+
+
 class AspectPropertyValue(object):
 
     """!
@@ -66,8 +71,8 @@ def value(self):
 
         """!
         Get the value out of the AspectPropertyValue wrapper
-        Note that this will return an error (including no_value_message if set)
-        if there is no value. So check has_value() first to be sure.
+        With no value this raises, with no_value_message or, when that is
+        not set, NO_VALUE_MESSAGE. So check has_value() first to be sure.
 
         @rtype mixed
         @return the stored value
@@ -77,7 +82,7 @@ def value(self):
         if self.__hasValue:
             return self.__value
         else:
-            raise Exception(self.no_value_message())
+            raise Exception(self.no_value_message() or NO_VALUE_MESSAGE)
 
     def no_value_message(self):
 

pipeline/fiftyone_pipeline_core/tests/test_aspectproperty_value.py

@@ -0,0 +1,62 @@
+# *********************************************************************
+# This Original Work is copyright of 51 Degrees Mobile Experts Limited.
+# Copyright 2026 51 Degrees Mobile Experts Limited, Davidson House,
+# Forbury Square, Reading, Berkshire, United Kingdom RG1 3EU.
+#
+# This Original Work is licensed under the European Union Public Licence
+# (EUPL) v.1.2 and is subject to its terms as set out below.
+#
+# If a copy of the EUPL was not distributed with this file, You can obtain
+# one at https://opensource.org/licenses/EUPL-1.2.
+#
+# The 'Compatible Licences' set out in the Appendix to the EUPL (as may be
+# amended by the European Commission) shall be deemed incompatible for
+# the purposes of the Work and the provisions of the compatibility
+# clause in Article 5 of the EUPL shall not apply.
+#
+# If using the Work as, or as part of, a network application, by
+# including the attribution notice(s) required under Article 5 of the EUPL
+# in the end user terms of the application under an appropriate heading,
+# such notice(s) shall fulfill the requirements of that article.
+# *********************************************************************
+
+import unittest
+
+from fiftyone_pipeline_core.aspectproperty_value import AspectPropertyValue
+
+NO_VALUE = "This instance does not have a set value"
+
+
+class AspectPropertyValueTests(unittest.TestCase):
+
+    """!
+    Reading the value of an AspectPropertyValue that has no value raises,
+    with the reason given or, when none was given, a message saying there
+    is no value, as the Null values section of properties.md in the
+    specifications asks. A value that is set, whatever it is, is read back
+    as it was set.
+    """
+
+    def test_reading_no_value_raises_with_a_message(self):
+        cases = [
+            ("no reason", AspectPropertyValue(), NO_VALUE),
+            ("a None reason", AspectPropertyValue(None), NO_VALUE),
+            ("an empty reason", AspectPropertyValue(""), NO_VALUE),
+            ("a reason", AspectPropertyValue("No match"), "No match"),
+        ]
+        for name, apv, message in cases:
+            with self.subTest(name):
+                self.assertFalse(apv.has_value())
+                with self.assertRaises(Exception) as raised:
+                    apv.value()
+                self.assertEqual(message, str(raised.exception))
+
+    def test_no_reason_leaves_the_message_unset(self):
+        self.assertIsNone(AspectPropertyValue().no_value_message())
+
+    def test_a_value_is_read_as_set(self):
+        for value in ["Value", None, False, 0, "", ["a", "b"]]:
+            with self.subTest(value=value):
+                apv = AspectPropertyValue(value=value)
+                self.assertTrue(apv.has_value())
+                self.assertEqual(value, apv.value())