Distinguish between nil and derived in validate.py

src/ifcwrap/IfcParseWrapper.i

@@ -54,7 +54,41 @@ private:
 %rename("add") addEntity;
 %rename("remove") removeEntity;
 
+class attribute_value_derived {};
 %{
+class attribute_value_derived {};
+%}
+
+%extend attribute_value_derived {
+	%pythoncode %{
+		def __bool__(self): return False
+		def __repr__(self): return '*'
+	%}
+}
+
+%inline %{
+static bool feature_use_attribute_value_derived = false;
+
+void set_feature(const std::string& x, PyObject* v) {
+	if (PyBool_Check(v) && x == "use_attribute_value_derived") {
+		feature_use_attribute_value_derived = v == Py_True;
+	} else {
+		throw std::runtime_error("Invalid feature specification");
+	}
+}
+
+PyObject* get_feature(const std::string& x) {
+	if (x == "use_attribute_value_derived") {
+		return PyBool_FromLong(feature_use_attribute_value_derived);
+	} else {
+		throw std::runtime_error("Invalid feature specification");
+	}
+}
+
+%}
+
+%{
+
 static const std::string& helper_fn_declaration_get_name(const IfcParse::declaration* decl) {
 	return decl->name();
 }

src/ifcwrap/utils/typemaps_out.i

@@ -38,9 +38,16 @@
 	try {
 	const Argument& arg = *($1.second);
 	const IfcUtil::ArgumentType type = $1.first;
-	if (arg.isNull() || type == IfcUtil::Argument_DERIVED) {
+	if (arg.isNull()) {
 		Py_INCREF(Py_None);
 		$result = Py_None;
+	} else if (type == IfcUtil::Argument_DERIVED) {
+		if (feature_use_attribute_value_derived) {
+			$result = SWIG_NewPointerObj(new attribute_value_derived, SWIGTYPE_p_attribute_value_derived, SWIG_POINTER_OWN);
+		} else {
+			Py_INCREF(Py_None);
+			$result = Py_None;
+		}
 	} else {
 	switch(type) {
 		case IfcUtil::Argument_INT: {