arbitrary property and function support for rna properties (arrays and collections), this means functions can be easily added.

eg.
 scene.objects.link()
 object.constraints.new()
 mesh.verts.transform(...)
 mesh.faces.active

PropertyRNA stores an StructRNA pointer where these can be defined.

source/blender/python/intern/bpy_operator_wrap.c

@@ -125,11 +125,11 @@ static int PYTHON_OT_generic(int mode, bContext *C, wmOperatorType *ot, wmOperat
 		}
 
 		RNA_pointer_create(NULL, &RNA_Context, C, &ptr_context);
-		
+
 		if (mode==PYOP_INVOKE) {
 			item= PyObject_GetAttrString(py_class, "invoke");
 			args = PyTuple_New(3);
-			
+
 			RNA_pointer_create(NULL, &RNA_Event, event, &ptr_event);
 
 			// PyTuple_SET_ITEM "steals" object reference, it is
@@ -149,9 +149,9 @@ static int PYTHON_OT_generic(int mode, bContext *C, wmOperatorType *ot, wmOperat
 			PyTuple_SET_ITEM(args, 1, pyrna_struct_CreatePyObject(&ptr_context));
 		}
 		PyTuple_SET_ITEM(args, 0, py_class_instance);
-	
+
 		ret = PyObject_Call(item, args, NULL);
-		
+
 		Py_DECREF(args);
 		Py_DECREF(item);
 		Py_DECREF(class_dict);