Note that this value is set during print preparation phase ([gtk.types.PrintStatus.Preparing]), so this value should never be get before the data generation phase ([gtk.types.PrintStatus.GeneratingData]). You can connect to the signal@Gtk.PrintOperation::status-changed signal and call [gtk.print_operation.PrintOperation.getNPagesToPrint] when print status is [gtk.types.PrintStatus.GeneratingData].
This is typically used to track the progress of print operation.