model.Extension: Change refers_to to be of type List[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 list 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_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

@@ -1458,7 +1458,7 @@ def __init__(self,
  name: NameType,
  value_type: Optional[DataTypeDefXsd] = None,
  value: Optional[ValueDataType] = None,
- refers_to: Iterable[ModelReference] = (),
+ refers_to: Optional[List[ModelReference]] = None,
  semantic_id: Optional[Reference] = None,
  supplemental_semantic_id: Iterable[Reference] = ()):
  super().__init__()
@@ -1468,7 +1468,10 @@ def __init__(self,
  self.value_type: Optional[DataTypeDefXsd] = value_type
  self._value: Optional[ValueDataType]
  self.value = value
- self.refers_to: Iterable[ModelReference] = refers_to
+ if refers_to is None:
+ self.refers_to: List[ModelReference] = []
+ else:
+ self.refers_to = refers_to
  self.semantic_id: Optional[Reference] = semantic_id
  self.supplemental_semantic_id: ConstrainedList[Reference] = ConstrainedList(supplemental_semantic_id)