Unassigning the last unit now cleans up empty unit assignments

src/ifcopenshell-python/ifcopenshell/api/unit/unassign_unit.py

@@ -14,4 +14,5 @@ class Usecase:
         units = units - set(self.settings["units"])
         if units:
             unit_assignment.Units = list(units)
-        return unit_assignment
+            return unit_assignment
+        self.file.remove(unit_assignment)

src/ifcopenshell-python/test/api/unit/test_unassign_unit.py

@@ -0,0 +1,24 @@
+import test.bootstrap
+import ifcopenshell.api
+
+
+class TestUnassignUnit(test.bootstrap.IFC4):
+    def test_run(self):
+        project = self.file.createIfcProject()
+        unit1 = ifcopenshell.api.run("unit.add_monetary_unit", self.file, currency="FOO")
+        unit2 = ifcopenshell.api.run("unit.add_monetary_unit", self.file, currency="BAR")
+        assignment = ifcopenshell.api.run("unit.assign_unit", self.file, units=[unit1, unit2])
+        ifcopenshell.api.run("unit.unassign_unit", self.file, units=[unit1])
+        assert unit1 not in assignment.Units
+        assert unit2 in assignment.Units
+
+    def test_unassigning_the_last_unit(self):
+        project = self.file.createIfcProject()
+        unit = ifcopenshell.api.run("unit.add_monetary_unit", self.file, currency="FOO")
+        ifcopenshell.api.run("unit.assign_unit", self.file, units=[unit])
+        ifcopenshell.api.run("unit.unassign_unit", self.file, units=[unit])
+        assert project.UnitsInContext is None
+
+    def test_doing_nothing_if_the_unit_is_not_assigned(self):
+        unit = ifcopenshell.api.run("unit.add_monetary_unit", self.file, currency="FOO")
+        assert ifcopenshell.api.run("unit.unassign_unit", self.file, units=[unit]) is None