Add docs to layer API

src/ifcopenshell-python/ifcopenshell/api/layer/assign_layer.py

@@ -20,14 +20,52 @@ import ifcopenshell
 
 
 class Usecase:
-    def __init__(self, file, **settings):
+    def __init__(self, file, item=None, layer=None):
+        """Assigns a representation item to a layer
+
+        In IFC, instead of objects being assigned to layers, representation
+        items are assigned to layers. Representation items are portions of the
+        object's representation. For example, this allows a single IFC Window
+        element to have portions of its 2D linework (e.g. the cross section of
+        its frame) assigned to one layer, and another portion (e.g. the glazing
+        panels) assigned to another layer.
+
+        :param item: The IfcRepresentationItem to assign to the layer. This
+            should be one of the items in the object's IfcShapeRepresentation.
+        :type item: ifcopenshell.entity_instance.entity_instance
+        :param layer: The IfcPresentationLayerAssignment layer to assign the
+            item to.
+        :type layer: ifcopenshell.entity_instance.entity_instance
+        :return: None
+        :rtype: None
+
+        Example::
+
+            # Remember, all geometry needs to specify the context it is part of first.
+            # See ifcopenshell.api.context.add_context for details.
+            model = ifcopenshell.api.run("context.add_context", model, context_type="Model")
+            body = ifcopenshell.api.run("context.add_context", model,
+                context_type="Model", context_identifier="Body", target_view="MODEL_VIEW", parent=model
+            )
+
+            wall = ifcopenshell.api.run("root.create_entity", model, ifc_class="IfcWall")
+            representation = ifcopenshell.api.run("geometry.add_wall_representation", model,
+                context=body, length=5, height=3, thickness=0.2)
+            ifcopenshell.api.run("geometry.assign_representation", model,
+                product=wall, representation=representation)
+
+            # Now let's create a layer that contains walls
+            layer = ifcopenshell.api.run("layer.add_layer", model, Name="AI-WALL")
+
+            # And assign our wall representation item (in this example, there is
+            # only one item) to the layer.
+            ifcopenshell.api.run("layer.assign_layer", model, item=representation.Items[0], layer=layer)
+        """
         self.file = file
         self.settings = {
-            "item": None,
-            "layer": None,
+            "item": item,
+            "layer": layer,
         }
-        for key, value in settings.items():
-            self.settings[key] = value
 
     def execute(self):
         assigned_items = set(self.settings["layer"].AssignedItems or [])