Drag and drop in QTreeView fails with items that hold a QImage - python-3.x

Question

I have a list of items in a QTreeView. Each item holds a QImage object. If I try to drag and drop the item, the program freezes. But when I comment out the line objMod._Image = QImage(flags = Qt.AutoColor), the program runs fine.
How can I drag and drop the items with the QImage object? The QImage holds an image which is rendered. The rendering process takes a while, so it would be nice to keep the QImage object.
import sys
import os
from PySide.QtCore import *
from PySide.QtGui import *
from PySide.QtUiTools import *
from PIL import Image, ImageCms, ImageQt
class ObjModel:
def __init__(self):
self._Image = None
class DragMoveTest(QMainWindow):
def __init__(self):
super(DragMoveTest,self).__init__()
self.initGUI()
self.show()
def initGUI(self):
self.treeView = QTreeView()
modelTreeView = QStandardItemModel()
self.treeView.setModel(modelTreeView)
for i in range(0, 4):
objMod = ObjModel()
objMod._Image = None
objMod._Image = QImage(flags = Qt.AutoColor)
item = QStandardItem('Test: %s' % str(i))
item.setData(objMod, Qt.UserRole + 1)
modelTreeView.invisibleRootItem().appendRow(item)
self.treeView.setDragDropMode(QAbstractItemView.InternalMove)
self.setCentralWidget(self.treeView)
def main(args):
app = QApplication(sys.argv)
qt_main_wnd = DragMoveTest()
ret = app.exec_()
sys.exit(ret)
if __name__ == "__main__":
main(sys.argv)

Answer 1

This is caused by a bug in PySide. During a drag and drop operation, the data in the dragged item must be serialized. This will be handled by Qt for most data-types, but for types that are specific to Python, special handling is required. This special handling seems to be broken in PySide. If your example is converted to PyQt, a TypeError is raised when trying to drag items, but the program does not freeze.
The source of the problem is that you are storing data using a custom Python class. PyQt uses pickle to serialize custom data-types, but it is not possible to also pickle the QImage that is stored in its __dict__, so the operation fails. I assume PySide must attempt something similar, but for some reason it does not raise an error when it fails. Qt grabs the mouse whilst dragging, so if the operation fails abnormally, it won't be released again, and the program will appear to freeze.
The simplest way to fix this is to avoid using a custom class to hold the QImage, and instead store the image directly in the item:
image = QImage()
item = QStandardItem('Test: %s' % i)
item.setData(image, Qt.UserRole + 1)
To store more data items, you can either use a different data-role for each one, or use a dict to hold them all:
data = {'image': QImage(), 'title': 'foo', 'timestamp': 1756790}
item.setData(data, Qt.UserRole + 1)
However, if you do this, you must always use string keys in the dict, otherwise you will face the same problems as before. (Using string keys means the dict can be converted into a QMap, which Qt knows how to serialize).
(NB: if you want to know whether a Qt class can be serialized, check the docs to see whether it defines the datastream operators).

Answer 2

I come up with a different solution. It is easier to have a object that holds a io.BytesIO. Your store the ImageData into the bytesIO variable. Upon on your image library you can open the image from the bytesIO variable.
In the demo the class ObjModel can handle QImage and Image from Pillow/PIL. If you use the set methods the image object will be converted into a bytesIO.
In short here a working example:
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import sys
import os
import io
from PySide.QtCore import *
from PySide.QtGui import *
from PySide.QtUiTools import *
from PIL import Image, ImageCms, ImageQt
########################################################################
class ObjModel:
""""""
#----------------------------------------------------------------------
def __init__(self):
"""Constructor"""
self._ImageByteIO = None
#----------------------------------------------------------------------
def getObjByte(self):
""""""
return self._ImageByteIO
#----------------------------------------------------------------------
def getQImage(self):
""""""
try:
self._ImageByteIO.seek(0)
qImg = QImage.fromData(self._ImageByteIO.getvalue())
return qImg
except:
return None
#----------------------------------------------------------------------
def getPILImage(self):
""""""
try:
self._ImageByteIO.seek(0)
img = Image.open(tBytesIO)
return img
except:
return None
#----------------------------------------------------------------------
def setObjByte(self, fileName):
""""""
try:
tBytesIO = io.BytesIO()
f = open (fileName, 'rb')
tBytesIO.write(f.read())
f.close()
self._ImageByteIO = tBytesIO
except:
self._ImageByteIO = None
#----------------------------------------------------------------------
def setQImage(self, qImg):
""""""
try:
tBytesIO = io.BytesIO()
qByteArray = QByteArray()
qBuf = QBuffer(qByteArray)
qBuf.open(QIODevice.ReadWrite)
qImg.save(qBuf, 'PNG')
tBytesIO = io.BytesIO()
tBytesIO.write(qByteArray.data())
self._ImageByteIO = tBytesIO
except:
self._ImageByteIO = None
#----------------------------------------------------------------------
def setPILImage(self, pImg):
""""""
tBytesIO = io.BytesIO()
pImg.save(tBytesIO, 'png')
self._ImageByteIO = tBytesIO
#----------------------------------------------------------------------
class DragMoveTest(QMainWindow):
def __init__(self):
""""""
super(DragMoveTest,self).__init__()
self.initGUI()
self.show()
#----------------------------------------------------------------------
def initGUI(self):
""""""
self.treeView = QTreeView()
modelTreeView = QStandardItemModel()
self.treeView.setModel(modelTreeView)
for i in range(0, 4):
objMod = ObjModel()
objMod.setQImage(QImage(flags = Qt.AutoColor))
item = QStandardItem('Test: %s' % str(i))
item.setData(objMod, Qt.UserRole + 1)
modelTreeView.invisibleRootItem().appendRow(item)
self.treeView.setDragDropMode(QAbstractItemView.InternalMove)
self.setCentralWidget(self.treeView)
#----------------------------------------------------------------------
def main(args):
app = QApplication(sys.argv)
qt_main_wnd = DragMoveTest()
ret = app.exec_()
sys.exit(ret)
#----------------------------------------------------------------------
if __name__ == "__main__":
main(sys.argv)