Fix #1576. Managing IfcSystem subtypes is now possible.

src/ifcopenshell-python/ifcopenshell/api/system/add_system.py

@@ -23,13 +23,13 @@ import ifcopenshell.api
 class Usecase:
     def __init__(self, file, **settings):
         self.file = file
-        self.settings = {}
+        self.settings = {"ifc_class": "IfcSystem"}
         for key, value in settings.items():
             self.settings[key] = value
 
     def execute(self):
         return self.file.create_entity(
-            "IfcSystem",
+            self.settings["ifc_class"],
             **{
                 "GlobalId": ifcopenshell.guid.new(),
                 "OwnerHistory": ifcopenshell.api.run("owner.create_owner_history", self.file),

src/ifcopenshell-python/ifcopenshell/util/system.py

@@ -0,0 +1,36 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2022 Dion Moult <dion@thinkmoult.com>
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+
+
+def get_system_elements(system):
+    results = []
+    for rel in system.IsGroupedBy:
+        results.extend(rel.RelatedObjects)
+    return results
+
+
+def get_element_systems(element):
+    results = []
+    for rel in element.HasAssignments:
+        if rel.is_a("IfcRelAssignsToGroup") and rel.RelatingGroup.is_a() in [
+            "IfcSystem",
+            "IfcDistributionSystem",
+            "IfcBuildingSystem",
+        ]:
+            results.append(rel.RelatingGroup)
+    return results