First attempt at ripping out agnostic code into ifcopenshell.api namespace. See #1399.

src/ifcopenshell-python/ifcopenshell/api/pset/add_pset.py

@@ -0,0 +1,40 @@
+import ifcopenshell
+import blenderbim.bim.schema  # TODO: refactor
+
+
+class Usecase:
+    def __init__(self, file, settings=None):
+        self.file = file
+        self.settings = {"product": None, "Name": None}
+        for key, value in settings.items():
+            self.settings[key] = value
+
+    def execute(self):
+        if self.settings["product"].is_a("IfcObject"):
+            pset = self.file.create_entity(
+                "IfcPropertySet", **{"GlobalId": ifcopenshell.guid.new(), "Name": self.settings["Name"]}
+            )
+            self.file.create_entity(
+                "IfcRelDefinesByProperties",
+                **{
+                    "GlobalId": ifcopenshell.guid.new(),
+                    # TODO: owner history
+                    "RelatedObjects": [self.settings["product"]],
+                    "RelatingPropertyDefinition": pset,
+                }
+            )
+        elif self.settings["product"].is_a("IfcTypeObject"):
+            pset = self.file.create_entity(
+                "IfcPropertySet", **{"GlobalId": ifcopenshell.guid.new(), "Name": self.settings["Name"]}
+            )
+            has_property_sets = list(self.settings["product"].HasPropertySets)
+            has_property_sets.append(pset)
+            self.settings["product"].HasPropertySets = has_property_sets
+        elif self.settings["product"].is_a("IfcMaterialDefinition"):
+            pset = self.file.create_entity(
+                "IfcMaterialProperties",
+                **{
+                    "Name": self.settings["Name"],
+                    "Material": self.settings["product"],
+                }
+            )