title: Drag and Drop of subclassed QListWidgetItem
body: <p>I'm encountering some poblems trying to implement drag and drop for a custom QListWidgetItem. Here is some example code:</p>
  
  <pre><code>from PyQt4 import QtGui, QtCore
  import sys, os
  
  
  class MyListWidgetItem(QtGui.QListWidgetItem):      
      def __init__(self, label, data, parent=None):
          super(QtGui.QListWidgetItem, self).__init__(label, parent=parent)
          self.data = data
  
      def GetData(self):
          return self.data
  
  class MyListWidget(QtGui.QListWidget):
      def __init__(self, type, parent=None):
          super(MyListWidget, self).__init__(parent)
          self.setDragDropMode(QtGui.QAbstractItemView.DragDrop)
          self.setSelectionMode(QtGui.QAbstractItemView.ExtendedSelection)
          self.setAcceptDrops(True)
          self.viewport().setAcceptDrops(True)
          self.setDropIndicatorShown(True)
  
      def startDrag(self, supportedActions):
          drag = QtGui.QDrag(self)
          t = [i.GetData() for i in self.selectedItems()]
          mimeData = self.model().mimeData(self.selectedIndexes())
          mimeData.setText(str(t))
          drag.setMimeData(mimeData)
          if drag.start(QtCore.Qt.MoveAction) == QtCore.Qt.MoveAction:
              for item in self.selectedItems():
                  self.takeItem(self.row(item))
  
      def dragEnterEvent(self, event):
          if event.mimeData().hasUrls():
              event.ignore()
          else:
              event.accept()
  
      def dragMoveEvent(self, event):
          if event.mimeData().hasUrls():
              event.ignore()
          else:
              event.accept()
  
      def dropEvent(self, event):
          if event.mimeData().hasUrls():
              event.ignore()
          if isinstance(event.source(), MyListWidget):
              event.setDropAction(QtCore.Qt.MoveAction)
              super(MyListWidget, self).dropEvent(event)
          else:
              event.ignore()
  
      def dropMimeData(self, index, mimedata, action):
          super(MyListWidget, self).dropMimeData(index, mimedata, action)
          return True
  
  class Test(QtGui.QMainWindow):
      def __init__(self):
          super(QtGui.QMainWindow,self).__init__()
          myQWidget = QtGui.QWidget()
          myBoxLayout = QtGui.QVBoxLayout()
          myQWidget.setLayout(myBoxLayout)
          self.setCentralWidget(myQWidget)
  
          self.listWidgetA = MyListWidget(self)
          self.listWidgetB = MyListWidget(self)
  
          for i in range(5):
              listItemAInstance = MyListWidgetItem(str(i), i, parent=self.listWidgetA)
  
          myBoxLayout.addWidget(self.listWidgetA)      
          myBoxLayout.addWidget(self.listWidgetB)   
  
  if __name__ == '__main__':
      app = QtGui.QApplication(sys.argv)
      dialog_1 = Test()
      dialog_1.show()
      dialog_1.resize(480,320)
      sys.exit(app.exec_())
  </code></pre>
  
  <p>My custom class MyListWidgetItem has a 'data' field which in my real program holds some information related to the item. If you run the code and try to drag and drop items from the top list to the bottom one everything works, but if you then try to bring them back to the top list you get this error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "C:\Users\Massi\Desktop\t.py", line 23, in startDrag
      t = [i.GetData() for i in self.selectedItems()]
  AttributeError: 'QListWidgetItem' object has no attribute 'GetData'
  </code></pre>
  
  <p>It seems pretty clear that the default drag and drop behaviour ignores that the list items have been subclassed, so I wonder which is the best to deal with this situation. Any help is really appreciated.
  Thanks in advance!</p>