pyside qtreewidget constrain drag and drop

Qt does not seem to make this sort of thing very easy.

The best I could come up with was to temporarily reset the item flags during the drag-enter and drag-move events. The example below calculates the current top-level item dynamically in order to contrain drag and drop. But it could also be done by using setData() to add an identifier to each item.

from PyQt4 import QtCore, QtGui

class TreeWidget(QtGui.QTreeWidget):
    def __init__(self, parent=None):
        QtGui.QTreeWidget.__init__(self, parent)
        self.setDragDropMode(self.InternalMove)
        self.setDragEnabled(True)
        self.setDropIndicatorShown(True)
        self._dragroot = self.itemRootIndex()

    def itemRootIndex(self, item=None):
        root = self.invisibleRootItem()
        while item is not None:
            item = item.parent()
            if item is not None:
                root = item
        return QtCore.QPersistentModelIndex(
            self.indexFromItem(root))

    def startDrag(self, actions):
        items = self.selectedItems()
        self._dragroot = self.itemRootIndex(items and items[0])
        QtGui.QTreeWidget.startDrag(self, actions)

    def dragEnterEvent(self, event):
        self._drag_event(event, True)

    def dragMoveEvent(self, event):
        self._drag_event(event, False)

    def _drag_event(self, event, enter=True):
        items = []
        disable = False
        item = self.itemAt(event.pos())
        if item is not None:
            disable = self._dragroot != self.itemRootIndex(item)
            if not disable:
                rect = self.visualItemRect(item)
                if event.pos().x() < rect.x():
                    disable = True
        if disable:
            for item in item, item.parent():
                if item is not None:
                    flags = item.flags()
                    item.setFlags(flags & ~QtCore.Qt.ItemIsDropEnabled)
                    items.append((item, flags))
        if enter:
            QtGui.QTreeWidget.dragEnterEvent(self, event)
        else:
            QtGui.QTreeWidget.dragMoveEvent(self, event)
        for item, flags in items:
            item.setFlags(flags)

class Window(QtGui.QWidget):
    def __init__(self):
        QtGui.QWidget.__init__(self)
        self.tree = TreeWidget(self)
        self.tree.header().hide()
        def add(root, *labels):
            item = QtGui.QTreeWidgetItem(self.tree, [root])
            item.setFlags(item.flags() &
                          ~(QtCore.Qt.ItemIsDragEnabled |
                            QtCore.Qt.ItemIsDropEnabled))
            for index, title in enumerate(
                ('BackgroundObjects', 'ForegroundObjects')):
                subitem = QtGui.QTreeWidgetItem(item, [title])
                subitem.setFlags(
                    subitem.flags() & ~QtCore.Qt.ItemIsDragEnabled)
                for text in labels[index].split():
                    child = QtGui.QTreeWidgetItem(subitem, [text])
                    child.setFlags(
                        child.flags() & ~QtCore.Qt.ItemIsDropEnabled)
        add('isDelicious', 'Durian', 'Apple Banana Carrot')
        add('isSmelly', 'Apple Carrot', 'Banana Durian')
        root = self.tree.invisibleRootItem()
        root.setFlags(root.flags() & ~QtCore.Qt.ItemIsDropEnabled)
        self.tree.expandAll()
        layout = QtGui.QVBoxLayout(self)
        layout.addWidget(self.tree)

if __name__ == '__main__':

    import sys
    app = QtGui.QApplication(sys.argv)
    window = Window()
    window.setGeometry(500, 300, 300, 300)
    window.show()
    sys.exit(app.exec_())