DiagLayer: always provide .diag_data_dictionary_spec in non-raw dia…

…g layers

If the underlying raw layer does not provide a DDDS, and inheritance
does not apply either, an empty object is created. This should help to
avoid having to handle a few special cases in user code.

Thanks to [at]kayoub for the catch!

Signed-off-by: Andreas Lauser <andreas.lauser@mbition.io>
Signed-off-by: Florian Jost <florian.jost@mbition.io>

odxtools/diaglayers/diaglayer.py

@@ -48,6 +48,28 @@ def from_et(et_element: ElementTree.Element, doc_frags: List[OdxDocFragment]) ->
         # Create DiagLayer
         return DiagLayer(diag_layer_raw=diag_layer_raw)
 
+    def __post_init__(self) -> None:
+        if self.diag_layer_raw.diag_data_dictionary_spec is None:
+            # create an empry DiagDataDictionarySpec object if the raw
+            # layer does not define a DDDS...
+            self._diag_data_dictionary_spec = DiagDataDictionarySpec(
+                admin_data=None,
+                data_object_props=NamedItemList(),
+                dtc_dops=NamedItemList(),
+                structures=NamedItemList(),
+                static_fields=NamedItemList(),
+                end_of_pdu_fields=NamedItemList(),
+                dynamic_endmarker_fields=NamedItemList(),
+                dynamic_length_fields=NamedItemList(),
+                tables=NamedItemList(),
+                env_data_descs=NamedItemList(),
+                env_datas=NamedItemList(),
+                muxs=NamedItemList(),
+                unit_spec=None,
+                sdgs=[])
+        else:
+            self._diag_data_dictionary_spec = self.diag_layer_raw.diag_data_dictionary_spec
+
     def _build_odxlinks(self) -> Dict[OdxLinkId, Any]:
         """Construct a mapping from IDs to all objects that are contained in this diagnostic layer."""
         result = self.diag_layer_raw._build_odxlinks()
@@ -242,9 +264,9 @@ def sdgs(self) -> List[SpecialDataGroup]:
         return self.diag_layer_raw.sdgs
 
     @property
-    def diag_data_dictionary_spec(self) -> Optional[DiagDataDictionarySpec]:
+    def diag_data_dictionary_spec(self) -> DiagDataDictionarySpec:
         """The DiagDataDictionarySpec applicable to this DiagLayer"""
-        return self.diag_layer_raw.diag_data_dictionary_spec
+        return self._diag_data_dictionary_spec
 
     #####
     # </properties forwarded to the "raw" diag layer>

odxtools/diaglayers/hierarchyelement.py

@@ -437,7 +437,7 @@ def not_inherited_fn(parent_ref: ParentRef) -> List[str]:
     # <properties subject to value inheritance>
     #######
     @property
-    def diag_data_dictionary_spec(self) -> Optional[DiagDataDictionarySpec]:
+    def diag_data_dictionary_spec(self) -> DiagDataDictionarySpec:
         return self._diag_data_dictionary_spec
 
     @property