You can pass a few simple data types and objects that are derived from vtkObject class by using @vtk.calldata_type decorator. See detailed information here: https://www.slicer.org/wiki/Documentation/Nightly/Developers/FAQ#How_can_I_access_callData_argument_in_a_VTK_object_observer_callback_function
@dgobbi is there a user documentation of VTK’s Python wrapper? I couldn’t find a relevant section in in VTK API documentation and full-text search is not enabled, so I couldn’t even see if “vtk.calldata_type” is mentioned.