如何拖放具有附加属性的 QListWidgetItem 的子 class 实例？

Question

I am using PyQt5 and quite new to it. I would like to drag and drop a QListWidgetItem from one QListWidget to the other, such that the resulting QListWidgetItem on one side will contain additional data. I tried sub-classing QListWidgetItem, but the type does not carry through drag and drop as it seems to create a new instance of QListWidgetItem.

Here is the code:

from PyQt5.QtWidgets import QApplication, QMainWindow, QWidget, QHBoxLayout, QListWidget, QListWidgetItem, QAction, QVBoxLayout
from PyQt5 import QtCore, QtGui
import sys


class GUI(QMainWindow):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.central_widget = MyCentralWidget(self)
        self.setCentralWidget(self.central_widget)


class MyCustomItem(QListWidgetItem):
    def __init__(self, data, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.extra_data = data


class MyCentralWidget(QWidget):
    def __init__(self, parent):
        super(MyCentralWidget, self).__init__(parent)
        self.h_layout = QHBoxLayout(self)
        self.setLayout(self.h_layout)

        self.list_left = DragAndDropList()
        self.list_left.setDragEnabled(True)
        self.list_left.setAcceptDrops(False)

        self.list_right = DragAndDropList()

        self.h_layout.addWidget(self.list_left)
        self.h_layout.addWidget(self.list_right)

        item = MyCustomItem(69, 'custom_item')
        self.list_left.insertItem(1, item)


class DragAndDropList(QListWidget):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.setIconSize(QtCore.QSize(124, 124))
        self.setDragDropMode(self.DragDrop)
        self.setSelectionMode(self.ExtendedSelection)
        self.setAcceptDrops(True)

    def dragEnterEvent(self, event):
        if event.mimeData().hasUrls():
            event.accept()
        else:
            super().dragEnterEvent(event)

    def dragMoveEvent(self, event):
        if event.mimeData().hasUrls():
            event.setDropAction(QtCore.Qt.CopyAction)
            event.accept()
        else:
            super().dragMoveEvent(event)

    def dropEvent(self, event):
        if event.mimeData().hasUrls():
            event.setDropAction(QtCore.Qt.CopyAction)
            event.accept()
            links = []
            for url in event.mimeData().urls():
                links.append(str(url.toLocalFile()))
            self.emit(QtCore.SIGNAL("dropped"), links)
        else:
            event.setDropAction(QtCore.Qt.LinkAction)
            super().dropEvent(event)

            list_items = [self.item(i) for i in range(self.count())]
            for l in list_items:
                print(l.extra_data)


def main():
    app = QApplication([])
    win = GUI()
    win.show()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

Here I get the error: "AttributeError: 'QListWidgetItem' object has no attribute 'extra_data'", when I try to drag and drop the custom_item.

I did have look at this thread [ 1 ], but it is out of date and there were no clear solutions.

Answer 1

Subclassing QListWidgetItem won't help you, as the drag and drop data is always serialized, which means that no instance reference is ever exchanged.

If your data is serializable (strings, obviously, but usually any QVariant type is fine, such as QColor or QPixmap), you can just use QListWidgetItem.setData() with a custom role specific for every data field you want to store, and then use QListWidgetItem.data() to get that data back.

In this case I created a custom function, but that's obviously not necessary, as you can just manually set the data for each item before or after inserting it, as long as you have the right row index.

MySpecialRole = QtCore.Qt.UserRole + 1

class DragAndDropList(QtWidgets.QListWidget):
    def addCustomItem(self, name, data):
        item = QtWidgets.QListWidgetItem(name)
        item.setData(MySpecialRole, data)
        self.addItem(item)

    def dropEvent(self, event):
        super().dropEvent(event)
        # let's see the data for each item, including our custom role
        for row in range(self.count()):
            item = self.item(row)
            print('Data for item {}: {}'.format(
                row + 1, item.data(MySpecialRole)))


class MyCentralWidget(QtWidgets.QWidget):
    def __init__(self, parent=None):
        super(MyCentralWidget, self).__init__(parent)
        self.h_layout = QtWidgets.QHBoxLayout(self)
        self.setLayout(self.h_layout)

        self.list_left = DragAndDropList()
        self.list_left.setDragEnabled(True)

        self.list_right = DragAndDropList()
        self.list_right.setAcceptDrops(True)

        self.h_layout.addWidget(self.list_left)
        self.h_layout.addWidget(self.list_right)

        self.list_left.addCustomItem('item (using addCustomItem)', 'some data')
        self.list_left.addItem('item without custom data')
        self.list_left.addItem('this item will have data')
        self.list_left.item(2).setData(MySpecialRole, 'some custom data')

Note that the following line will give you an error:

    self.emit(QtCore.SIGNAL("dropped"), links)

Not only the list widget doesn't have an emit attribute (like any other subclass of QObject doesn't), but also custom signals have to be declared in the class definition and then emitted directly.

class DragAndDropList(QtWidgets.QListWidget):
    dropped = QtCore.pyqtSignal(object)
    def dropEvent(self, event):
        # ...
        self.dropped.emit(links)