PyQT QListWidget拖放不起作用

from PyQt5.QtWidgets import QApplication, QListWidget, QListWidgetItem, QMainWindow
from PyQt5.QtCore import Qt, QMimeData
from PyQt5.QtGui import QDrag

class MyListWidget(QListWidget):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.setAcceptDrops(True)
        self.setDragEnabled(True)
        self.setDragDropMode(QListWidget.InternalMove)

    def dragEnterEvent(self, event):
        if event.mimeData().hasFormat('text/plain'):
            event.accept()
        else:
            event.ignore()

    def dragMoveEvent(self, event):
        if event.mimeData().hasFormat('text/plain'):
            event.setDropAction(Qt.MoveAction)
            event.accept()
        else:
            event.ignore()

    def dropEvent(self, event):
        if event.mimeData().hasFormat('text/plain'):
            data = event.mimeData().text()
            item = QListWidgetItem(data, self)
            item.setFlags(Qt.ItemIsSelectable | Qt.ItemIsEnabled)
            event.setDropAction(Qt.MoveAction)
            event.accept()
        else:
            event.ignore()

if __name__ == '__main__':
    app = QApplication([])
    window = QMainWindow()
    list_widget = MyListWidget(window)
    list_widget.addItem("Item 1")
    list_widget.addItem("Item 2")
    list_widget.addItem("Item 3")
    window.setCentralWidget(list_widget)
    window.show()
    app.exec_()