model.Extension: Change refers_to to be of type Set[Reference]

Currently, `Extension.refers_to` is declared as a `Iterable[Reference]`.
This implies, that we can not necessarily check, whether or not the
attribute is empty or not.
This creates a problem with the XML serialization, since the
`<aas.refersTo>` element should only appear if there is at least
one `Reference` inside.

This commit changes the `Extension.refers_to` to be a set of
`Reference`s, as well as adapting a more clear check whether
or not the attribute is empty in `adapter.xml.xml_serialization`.

basyx/aas/adapter/xml/xml_deserialization.py

@@ -692,10 +692,11 @@ def construct_extension(cls, element: etree.Element, object_class=model.Extensio
         value = _get_text_or_none(element.find(NS_AAS + "value"))
         if value is not None:
             extension.value = model.datatypes.from_xsd(value, extension.value_type)
-        extension.refers_to = _failsafe_construct_multiple(
-            element.find(NS_AAS + "refersTo").findall(NS_AAS + "reference"),
-            cls._construct_referable_reference, cls.failsafe
-        )
+        refers_to = element.find(NS_AAS + "refersTo")
+        if refers_to is not None:
+            for ref in _child_construct_multiple(refers_to, NS_AAS + "reference", cls._construct_referable_reference,
+                                                 cls.failsafe):
+                extension.refers_to.add(ref)
         cls._amend_abstract_attributes(extension, element)
         return extension
 

basyx/aas/adapter/xml/xml_serialization.py

@@ -277,7 +277,7 @@ def extension_to_xml(obj: model.Extension, tag: str = NS_AAS+"extension") -> etr
                                               text=model.datatypes.XSD_TYPE_NAMES[obj.value_type]))
     if obj.value:
         et_extension.append(_value_to_xml(obj.value, obj.value_type))  # type: ignore # (value_type could be None)
-    if obj.refers_to:
+    if len(obj.refers_to) > 0:
         refers_to = _generate_element(NS_AAS+"refersTo")
         for reference in obj.refers_to:
             refers_to.append(reference_to_xml(reference, NS_AAS+"reference"))

basyx/aas/examples/data/example_aas.py

@@ -101,9 +101,9 @@ def create_example_asset_identification_submodel() -> model.Submodel:
         name='ExampleExtension',
         value_type=model.datatypes.String,
         value="ExampleExtensionValue",
-        refers_to=(model.ModelReference((model.Key(type_=model.KeyTypes.ASSET_ADMINISTRATION_SHELL,
+        refers_to=[model.ModelReference((model.Key(type_=model.KeyTypes.ASSET_ADMINISTRATION_SHELL,
                                                    value='http://acplt.org/RefersTo/ExampleRefersTo'),),
-                                        model.AssetAdministrationShell),))
+                                        model.AssetAdministrationShell)],)
 
     # Property-Element conform to 'Verwaltungssschale in der Praxis' page 41 ManufacturerName:
     # https://www.plattform-i40.de/PI40/Redaktion/DE/Downloads/Publikation/2019-verwaltungsschale-in-der-praxis.html

basyx/aas/model/base.py

@@ -1468,7 +1468,7 @@ def __init__(self,
         self.value_type: Optional[DataTypeDefXsd] = value_type
         self._value: Optional[ValueDataType]
         self.value = value
-        self.refers_to: Iterable[ModelReference] = refers_to
+        self.refers_to: Set[ModelReference] = set(refers_to)
         self.semantic_id: Optional[Reference] = semantic_id
         self.supplemental_semantic_id: ConstrainedList[Reference] = ConstrainedList(supplemental_semantic_id)