Widget inside QListWidgetItem disappears after internal move
I have a QListWidget which is populated by QLabel via .setItemWidget() and a drag and drop mode InternalMove, when I move an item inside the list its label disappears.
How can I solve this issue?
A minimal example to reproduce
from PyQt5.QtWidgets import (
QApplication, QLabel, QStyle,
QListWidget, QListWidgetItem
)
from PyQt5.QtCore import QSize
import sys
if __name__ == '__main__':
app = QApplication(sys.argv)
list = QListWidget()
list.setFixedHeight(400)
list.setDragDropMode(QListWidget.DragDropMode.InternalMove)
for _ in range(8):
item = QListWidgetItem()
item.setSizeHint(QSize(40, 40))
list.addItem(item)
label = QLabel()
label.setPixmap(list.style().standardIcon(
QStyle.StandardPixmap.SP_ArrowUp).pixmap(QSize(40,40)))
list.setItemWidget(item, label)
list.show()
sys.exit(app.exec())
edit
After reading the documentation for the .setItemWidget() which states:
This function should only be used to display static content in the place of a list widget item. If you want to display custom dynamic content or implement a custom editor widget, use QListView and subclass QStyledItemDelegate instead.
I wonder if this is related to the issue and what does "static content" mean in this context, is QLabel considered "dynamic content"?
edit #2
The problem is inside a dropEvent() a dropMimeData() is called which in turn creates a complete new item? (rowsInserted is called), which isn't supposed to happen for self items I guess, because a widget set in the dragged item isn't serialized and stored inside mimedata so the widget is decoupled, The dropMimeData() is usually called when you drag and drop items from a different list.
So I guess an ugly way to solve this is to store a manually serialized widget inside a QListWidget.mimeData() as a custom mimetype via QMimeData.setData() and recreate the widget after a drop inside QListWidget.dropMimeData().
for example:
from PyQt5.QtWidgets import (
QApplication, QLabel, QStyle,
QListWidget, QListWidgetItem
)
from PyQt5.QtCore import QSize, QMimeData, QBuffer, QIODevice
from PyQt5.QtGui import QPixmap
import pickle
import sys
class ListWidget(QListWidget):
def mimeData(self, items:list[QListWidgetItem]) -> QMimeData:
mimedata = QListWidget.mimeData(self, items)
# e.g. serialize pixmap
custommime = []
for item in items:
label:QLabel = self.itemWidget(item)
buff = QBuffer()
buff.open(QIODevice.OpenModeFlag.WriteOnly)
label.pixmap().save(buff, 'PNG')
buff.close()
custommime.append(buff.data())
mimedata.setData('application/custommime', pickle.dumps(custommime))
#
return mimedata
def dropMimeData(self, index:int, mimedata:QMimeData, action) -> bool:
result = QListWidget.dropMimeData(self, index, mimedata, action)
# e.g. recreate pixmap
if mimedata.hasFormat('application/custommime'):
for i, data in enumerate(
pickle.loads(mimedata.data('application/custommime')),
start=index):
pixmap = QPixmap()
pixmap.loadFromData(data, 'PNG')
label = QLabel()
label.setPixmap(pixmap)
self.setItemWidget(self.item(i), label)
#
return result
if __name__ == '__main__':
app = QApplication(sys.argv)
list = ListWidget()
list.setFixedHeight(400)
list.setDragDropMode(QListWidget.DragDropMode.InternalMove)
list.setSelectionMode(QListWidget.SelectionMode.ExtendedSelection)
for i in range(8):
item = QListWidgetItem()
item.setSizeHint(QSize(40, 40))
list.addItem(item)
label = QLabel()
label.setPixmap(list.style().standardIcon(
QStyle.StandardPixmap.SP_DialogOkButton + i).pixmap(QSize(40,40)))
list.setItemWidget(item, label)
list.show()
sys.exit(app.exec())
UPDATE
The bug has now been fixed in the latest versions of Qt5 and Qt6.
This is caused by a Qt bug which only affects fairly recent versions. I can consistently reproduce it when using Qt-5.15.6 and Qt-6.4.0 - but not e.g. Qt-5.12.1. The issue seems to be closely related to QTBUG-100128.
A work-around for PyQt5/6 (based on the solution by PaddleStroke) is as follows:
class ListWidget(QListWidget):
def dragMoveEvent(self, event):
if ((target := self.row(self.itemAt(event.pos()))) ==
(current := self.currentRow()) + 1 or
(current == self.count() - 1 and target == -1)):
event.ignore()
else:
super().dragMoveEvent(event)
OLD ANSWER:
Unfortunately, after some further experimentation today, it seems the suggested work-around given below isn't an effective solution. I have found it's also possible to make item-widgets disappear by drag and drop onto non-empty areas.
After testing some other versions of Qt5, I can confirm that the bug is completely absent in 5.12.x, 5.13.x, 5.14.x, 5.15.0 and 5.15.1. This agrees with the existing Qt bug report above which identified Qt-5.15.2 as the version where the bug was introduced.
Contrary to what is suggested in the question, there's no reason whatsoever why a label should not be used as an item-widget. The term "static content", just means "not updated by user-defined custom drawing".
This bug seems to be a regression from QTBUG-87057, which made quite a large number of internal changes to how list-view rows are moved during drag and drop. The complexity of those changes may mean a simple work-around that undoes its negative side-effects isn't possible. The changes affect all Qt5 versions greater than 5.15.1 and Qt6 versions greater than 6.0.
class ListWidget(QListWidget):
def dropEvent(self, event):
if (self.currentRow() < self.count() - 1 or
self.itemAt(event.pos()) is not None):
super().dropEvent(event)
list = ListWidget()
...
or using an event-filter:
class Monitor(QObject):
def eventFilter(self, source, event):
if event.type() == QEvent.Drop:
view = source.parent()
if (view.currentRow() == view.count() - 1 and
view.itemAt(event.pos()) is None):
return True
return super().eventFilter(source, event)
monitor = Monitor()
list = QListWidget()
list.viewport().installEventFilter(monitor)
...