Add documentation for cost module

src/ifcopenshell-python/ifcopenshell/api/cost/unassign_cost_item_quantity.py

@@ -20,11 +20,52 @@ import ifcopenshell.api
 
 
 class Usecase:
-    def __init__(self, file, **settings):
+    def __init__(self, file, cost_item=None, products=None):
+        """Removes quantities of a cost item that are calculated on products
+
+        A cost item may have quantities that are parametrically calculated on
+        physical products. This lets you remove those quantities. This means
+        that any future changes in the physical product's dimensions will not
+        have any impact on the cost item.
+
+        :param cost_item: The IfcCostItem to remove quantities from
+        :type cost_item: ifcopenshell.entity_instance.entity_instance
+        :param products: A list of IfcProducts that may have parametrically
+            connected quantities to the cost item
+        :type products: list[ifcopenshell.entity_instance.entity_instance]
+        :return: None
+        :rtype: None
+
+        Example::
+
+            schedule = ifcopenshell.api.run("cost.add_cost_schedule", model)
+            item = ifcopenshell.api.run("cost.add_cost_item", model, cost_schedule=schedule)
+
+            # Let's imagine a unit cost of 5.0 per unit volume
+            value = ifcopenshell.api.run("cost.add_cost_value", model, parent=item)
+            ifcopenshell.api.run("cost.edit_cost_value", model, cost_value=value,
+                attributes={"AppliedValue": 5.0})
+
+            slab = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcSlab")
+            # Usually the quantity would be automatically calculated via a
+            # graphical authoring application but let's assign a manual quantity
+            # for now.
+            qto = ifcopenshell.api.run("pset.add_qto", model, product=slab, name="Qto_SlabBaseQuantities")
+            ifcopenshell.api.run("pset.edit_qto", model, qto=qto, properties={"NetVolume": 42.0})
+
+            # Now let's parametrically link the slab's quantity to the cost
+            # item. If the slab is edited in the future and 42.0 changes, then
+            # the updated value will also automatically be applied to the cost
+            # item.
+            ifcopenshell.api.run("cost.assign_cost_item_quantity", model,
+                cost_item=item, products=[slab], prop_name="NetVolume")
+
+            # Let's change our mind and remove the parametric connection
+            ifcopenshell.api.run("cost.unassign_cost_item_quantity", model,
+                cost_item=item, products=[slab])
+        """
         self.file = file
-        self.settings = {"cost_item": None, "products": []}
-        for key, value in settings.items():
-            self.settings[key] = value
+        self.settings = {"cost_item": cost_item, "products": products or []}
 
     def execute(self):
         self.quantities = set(self.settings["cost_item"].CostQuantities or [])