取消Gtk.TreeView中某些特定项目的拖放

我有一个 Gtk.TreeView 在这里。大多数(但不是所有)项目都应该能够拖放。在本例中,第一项应该 不 可以拖放,但应该是可选的。

我怎么知道?也许我得用 drag-begin 发出信号,停止那里的拖拽。但我不知道怎么做。

#!/usr/bin/env python3
import gi
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk
from gi.repository import Gdk

class MainWindow(Gtk.Window):
    def __init__(self):
        Gtk.Window.__init__(self, title="TreeView Drag and Drop")
        self.connect("delete-event", Gtk.main_quit)
        self.set_default_size(400, 300)

        # "model" with dummy data
        self.store = Gtk.TreeStore(str)
        self.store.append(None, ['do not drag this'])
        self.store.append(None, ['drag this'])
        self.view = Gtk.TreeView(model=self.store)
        self.add(self.view)

        # build columsn
        colA = Gtk.TreeViewColumn('Col A', Gtk.CellRendererText(), text=0)
        self.view.append_column(colA)

        # DnD events
        self.view.connect("drag-data-received", self.drag_data_received)
        self.view.connect("drag-data-get", self.drag_data_get)
        self.view.connect("drag-begin", self.drag_begin)

        target_entry = Gtk.TargetEntry.new('text/plain', 2, 0)
        self.view.enable_model_drag_source(
                Gdk.ModifierType.BUTTON1_MASK,[target_entry], 
                Gdk.DragAction.DEFAULT|Gdk.DragAction.MOVE
        )
        self.view.enable_model_drag_dest(
                [target_entry],
                Gdk.DragAction.DEFAULT|Gdk.DragAction.MOVE
        )

    def drag_data_get (self, treeview, drag_context, data, info, time):
        model, path = treeview.get_selection().get_selected_rows()
        print('dd-get\tpath: {}'.format(path))
        data.set_text(str(path[0]), -1)

    def drag_data_received (self, treeview, drag_context, x,y, data,info, time):
        print('dd-received')
        store = treeview.get_model()
        source_iter = store.get_iter(data.get_text())
        dest_path, drop_pos = self.view.get_dest_row_at_pos(x, y)
        print('path: {} pos: {}'.format(dest_path, drop_pos))

    def drag_begin(self, widget, context):
        print(widget)
        print(context)

win = MainWindow()
win.show_all()
Gtk.main()