How to Drag and Drop from One QListWidget to Another

Question

There are two QListWIdgets sitting in a same dialog window. The DragDrop functionality has been enabled for both. If I drag and drop a file to any of two ListWidges the program recognizes it and prints out the list of the files dropped. But aside from drag and dropping files I would like to be able to drag and drop the List widget Items from one to another. If I drag the ListItems the drag and drop event is triggered. But it is not able to recognize what Items were dropped onto the widget. The example code is below. The goal is to drag-drop the list items from one ListWidget to another.

import sys, os
from PyQt4 import QtCore, QtGui   
class ThumbListWidget(QtGui.QListWidget):
    def __init__(self, type, parent=None):
        super(ThumbListWidget, self).__init__(parent)
        self.setAcceptDrops(True)
        self.setIconSize(QtCore.QSize(124, 124))

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

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

    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.ignore()


class Dialog_01(QtGui.QMainWindow):
    def __init__(self):
        super(QtGui.QMainWindow,self).__init__()
        self.listItems={}

        myQWidget = QtGui.QWidget()
        myBoxLayout = QtGui.QVBoxLayout()
        myQWidget.setLayout(myBoxLayout)
        self.setCentralWidget(myQWidget)

        self.listWidgetA = ThumbListWidget(self)
        for i in range(12): 
            QtGui.QListWidgetItem( 'Item '+str(i), self.listWidgetA )
        myBoxLayout.addWidget(self.listWidgetA)

        self.listWidgetB = ThumbListWidget(self)
        myBoxLayout.addWidget(self.listWidgetB)   

        self.listWidgetA.setDragDropMode(QtGui.QAbstractItemView.InternalMove)
        self.listWidgetA.setSelectionMode(QtGui.QAbstractItemView.ExtendedSelection)
        self.connect(self.listWidgetA, QtCore.SIGNAL("dropped"), self.items_dropped)
        self.listWidgetA.currentItemChanged.connect(self.item_clicked)

        self.listWidgetB.setDragDropMode(QtGui.QAbstractItemView.InternalMove)
        self.listWidgetB.setSelectionMode(QtGui.QAbstractItemView.ExtendedSelection)
        self.connect(self.listWidgetB, QtCore.SIGNAL("dropped"), self.items_dropped)
        self.listWidgetB.currentItemChanged.connect(self.item_clicked)

    def items_dropped(self, arg):
        print arg

    def item_clicked(self, arg):
        print arg

if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    dialog_1 = Dialog_01()
    dialog_1.show()
    dialog_1.resize(480,320)
    sys.exit(app.exec_())

EDIT # 2

Here is the code that does it all. But there is no way to track down what object was dropped. The droppedOnA() and droppedOnB() methods are still not working.

---

from PyQt4 import QtGui, QtCore
import sys, os


class MyClassItem(QtGui.QListWidgetItem):
    def __init__(self, parent=None):
        super(QtGui.QListWidgetItem, self).__init__(parent)       


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

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

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

    def dropEvent(self, event):
        print 'dropEvent', 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.MoveAction)
            super(ThumbListWidget, self).dropEvent(event)


class Dialog_01(QtGui.QMainWindow):
    def __init__(self):
        super(QtGui.QMainWindow,self).__init__()
        self.listItems={}

        myQWidget = QtGui.QWidget()
        myBoxLayout = QtGui.QVBoxLayout()
        myQWidget.setLayout(myBoxLayout)
        self.setCentralWidget(myQWidget)

        self.listWidgetA = ThumbListWidget(self)
        self.listWidgetB = ThumbListWidget(self)

        for i in range(7):
            listItemAInstance=MyClassItem()
            listItemAInstance.setText('A'+'%04d'%i)
            listItemAInstance.setBackgroundColor(QtCore.Qt.darkGray)   
            if i%2: listItemAInstance.setBackgroundColor(QtCore.Qt.gray)
            self.listWidgetA.addItem(listItemAInstance) 

            listItemBInstance=MyClassItem()
            listItemBInstance.setText('B'+'%04d'%i)

            if i%2: listItemBInstance.setBackgroundColor(QtCore.Qt.lightGray)
            self.listWidgetB.addItem(listItemBInstance) 

        myBoxLayout.addWidget(self.listWidgetA)      

        myBoxLayout.addWidget(self.listWidgetB)   
        self.connect(self.listWidgetA, QtCore.SIGNAL("dropped"), self.droppedOnA)
        self.connect(self.listWidgetB, QtCore.SIGNAL("dropped"), self.droppedOnB)


    def droppedOnA(self, arg):
        print '\n\t droppedOnA', arg.text

    def droppedOnB(self, arg):
        print '\n\t droppedOnB', arg.text


if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    dialog_1 = Dialog_01()
    dialog_1.show()
    dialog_1.resize(480,320)
    sys.exit(app.exec_())

EDIT # 3

Here is another attempt this time using MIME to pass dropped Item's objects to ListWidget. Unfortunately cPickle refuses to accept binary objects throwing a

TypeError: the sip.wrapper type cannot be instantiated or sub-classed

To get around it I convert each object names to string and use it with self.listItems={} dictionary as its key to retrieve list Item's binary objects. Which seems to be working well. But at the end when I almost though it as all done, a ListWidget with no visible errors doesn't add the dropped list Item to itself... It's strange.

self.listWidgetB.addItem(droppedItemInstance)

.

---

from PyQt4 import QtGui, QtCore
import sys, os
import cPickle 

class MyClassItem(QtGui.QListWidgetItem):
    def __init__(self, parent=None):
        super(QtGui.QListWidgetItem, self).__init__(parent)       


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

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

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

        else:
            event.setDropAction(QtCore.Qt.MoveAction)
            super(ThumbListWidget, self).dropEvent(event)


    def mimeTypes(self):
        return ['bstream', 'text/xml']

    def mimeData(self, droppedItems):
        mimedata = QtCore.QMimeData()

        droppedItemsAsStrings=[]
        for each in droppedItems:            
            droppedItemsAsStrings.append( str(each) )

        bstream = cPickle.dumps(droppedItemsAsStrings)
        mimedata.setData('bstream', bstream)        
        return mimedata


    def dropMimeData(self, action, mimedata, row):

        if action == QtCore.Qt.IgnoreAction: return True  

        dropped=cPickle.loads(str(mimedata.data('bstream')))

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

        return True


class Dialog_01(QtGui.QMainWindow):
    def __init__(self):
        super(QtGui.QMainWindow,self).__init__()
        self.listItems={}

        myQWidget = QtGui.QWidget()
        myBoxLayout = QtGui.QVBoxLayout()
        myQWidget.setLayout(myBoxLayout)
        self.setCentralWidget(myQWidget)

        self.listWidgetA = ThumbListWidget(self)
        self.listWidgetB = ThumbListWidget(self)

        for i in range(7):
            listItemAInstance=MyClassItem()
            listItemAInstance.setText('A'+'%04d'%i)
            listItemAInstance.setBackgroundColor(QtCore.Qt.darkGray)   
            if i%2: listItemAInstance.setBackgroundColor(QtCore.Qt.gray)
            self.listWidgetA.addItem(listItemAInstance) 

            listItemBInstance=MyClassItem()
            listItemBInstance.setText('B'+'%04d'%i)

            if i%2: listItemBInstance.setBackgroundColor(QtCore.Qt.lightGray)
            self.listWidgetB.addItem(listItemBInstance) 

            self.listItems[str(listItemAInstance)]=listItemAInstance
            self.listItems[str(listItemBInstance)]=listItemBInstance

        myBoxLayout.addWidget(self.listWidgetA)      

        myBoxLayout.addWidget(self.listWidgetB)   
        self.connect(self.listWidgetA, QtCore.SIGNAL("dropped"), self.droppedOnA)
        self.connect(self.listWidgetB, QtCore.SIGNAL("dropped"), self.droppedOnB)


    def droppedOnA(self, droppedItemsAsStrings):
        print '\n\t droppedOnA()'
        for each in droppedItemsAsStrings:
            if each in self.listItems.keys():
                droppedItemInstance = self.listItems[each]
                print 'adding', droppedItemInstance.text()
                self.listWidgetA.addItem(droppedItemInstance) 


    def droppedOnB(self, droppedItemsAsStrings):
        print '\n\t droppedOnB()'
        for each in droppedItemsAsStrings:
            if each in self.listItems.keys():
                droppedItemInstance = self.listItems[each]
                self.listWidgetB.addItem(droppedItemInstance) 
                print 'adding', droppedItemInstance.text()


if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    dialog_1 = Dialog_01()
    dialog_1.show()
    dialog_1.resize(480,320)
    sys.exit(app.exec_())

Geochronology

Answer 1

Here is a revised code. It is working like a charm! Bravo!

from PyQt4 import QtGui, QtCore
import sys, os

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

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

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

    def dropEvent(self, event):
        print 'dropEvent', 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.MoveAction)
            super(ThumbListWidget, self).dropEvent(event)


class Dialog_01(QtGui.QMainWindow):
    def __init__(self):
        super(QtGui.QMainWindow,self).__init__()
        self.listItems={}

        myQWidget = QtGui.QWidget()
        myBoxLayout = QtGui.QVBoxLayout()
        myQWidget.setLayout(myBoxLayout)
        self.setCentralWidget(myQWidget)

        self.listWidgetA = ThumbListWidget(self)
        for i in range(12): 
            QtGui.QListWidgetItem( 'Item '+str(i), self.listWidgetA )
        myBoxLayout.addWidget(self.listWidgetA)

        self.listWidgetB = ThumbListWidget(self)
        myBoxLayout.addWidget(self.listWidgetB)   

        self.connect(self.listWidgetA, QtCore.SIGNAL("dropped"), self.items_dropped)
        self.listWidgetA.currentItemChanged.connect(self.item_clicked)

        self.connect(self.listWidgetB, QtCore.SIGNAL("dropped"), self.items_dropped)
        self.listWidgetB.currentItemChanged.connect(self.item_clicked)

    def items_dropped(self, arg):
        print 'items_dropped', arg

    def item_clicked(self, arg):
        print arg

if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    dialog_1 = Dialog_01()
    dialog_1.show()
    dialog_1.resize(480,320)
    sys.exit(app.exec_())

Geochronology

Answer 2

Assuming you want to move the list-widget items, your subclass should be as below (note that setDragDropMode and setSelectionMode have been moved into __init__):

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

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

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

Albumin

Let me try it! Thanks! – Geochronology

Works great! Thanks Ekhumoro! – Geochronology

Apparently a miracle happens because of following lines: event.setDropAction(QtCore.Qt.MoveAction) and super(ThumbListWidget, self).dropEvent(event) I run the code with those lines commented out. These lines if removed cancel a drop action as well as an item assignment to the listWidget. Yet, I can't understand its syntax. I would appreciate if you clarify what happens there.... – Geochronology

The first line ensures the items are moved, rather than copied (which is the deafault). The second line calls the base-class implementation of dropEvent. A QListWidget already knows hows to copy/move its items, so we just let it do its thing... – Albumin

PS: If you comment out all the drag/drop methods, and set the default drag action to MoveAction, you'll end up with the same behaviour. It's only because you're overriding those methods for other reasons that those extra lines are needed. – Albumin

Please take a look at Edit # 2... since it appears we are not able to track the object that was dropped. After the Item dragged from ListA to ListB it is properly re-positioned in ListWidget on which it was dropped. That's great. But droppedOnA() and droppedOnB() functions are not triggered by dropped event. How that could be nailed down? – Geochronology

@Sputnix. You will have to reimplement startDrag and save a list of the currently selected indexes. Then in dropEvent you can check the saved list before clearing it for the next drag operation. – Albumin

Answer 3

The above code did not work for me, when implementing the Python 3 style of super().

The issue is that the inherited methods dragMoveEvent and dragDropEvent were not being overwritten with the new definitions in the child class.

Therefore event.setDropAction(QtCore.Qt.MoveAction) was not being called, and the widget behavior defaulted to QtCore.Qt.CopyAction.

I solved this using the setDefaultDropAction() method inherited from the QAbstractItemView class.

Here is the implementation in PyQt5 and Python 3.7:

class NewDragDropWidget(QListWidget):

    def __init__(self):
        super().__init__()
        self.setIconSize(QtCore.QSize(124, 124))
        self.setDragDropMode(QAbstractItemView.DragDrop)
        self.setDefaultDropAction(QtCore.Qt.MoveAction) # this was the magic line
        self.setSelectionMode(QAbstractItemView.ExtendedSelection)
        self.setAcceptDrops(True)

Ladew