为什么Maya 2009 TreeView控件在拖动时出现语法错误?

class View(event.Dispatcher):
    def __init__(self):
        self.window = cmds.window()
        tree_view = cmds.treeView(
            numberOfButtons=1,
            allowReparenting=True,
            dragAndDropCommand=self.tree_view_onDrag
        )
        cmds.showWindow(self.window)

    def tree_view_onDrag(self, dropped_items, old_parents, old_indexes, new_parent, new_indexes, item_before, item_after, *args):
        print "worked"

<bound method View.tree_view_onDrag of  {"layer 3"} {""} {1} "layer 1"  {0} "" "layer 2";

// Error: <bound method View.tree_view_onDrag of  {"layer 3"} {""} {1} "layer 1"  {0}€ // 
// Error: Line 1.1: Syntax error // 

编辑: