TreeView.setReorderable

This function is a convenience function to allow you to reorder models that support the [gtk.tree_drag_source_iface.TreeDragSourceIface] and the [gtk.tree_drag_dest_iface.TreeDragDestIface]. Both [gtk.tree_store.TreeStore] and [gtk.list_store.ListStore] support these. If reorderable is true, then the user can reorder the model by dragging and dropping rows. The developer can listen to these changes by connecting to the model’s GtkTreeModel::row-inserted and GtkTreeModel::row-deleted signals. The reordering is implemented by setting up the tree view as a drag source and destination. Therefore, drag and drop can not be used in a reorderable view for any other purpose.

This function does not give you any degree of control over the order -- any reordering is allowed. If more control is needed, you should probably handle drag and drop manually.