How to delete QTreeWidgetItem

Question

Several webpages say that QTreeWidgetItem can be deleted by deleting or QTreeWidget.clearing. But my code sample below doesn't seem to do so. Am I doing anything wrong?

#!/usr/bin/python
import sys
from PySide.QtGui import QApplication, QWidget, QTreeWidget, QTreeWidgetItem
#from PyQt4.QtGui import QApplication, QWidget, QTreeWidget, QTreeWidgetItem # Result was the same with `PySide`
import time

class TreeWidgetItemChild(QTreeWidgetItem):
    def __init__(self):
        super(TreeWidgetItemChild, self).__init__()
        print 'TreeWidgetItemChild init'

    def __del__(self):
        print 'TreeWidgetItemChild del'

def test_QTree_clear_children():
    tree = QTreeWidget()
    tree.setHeaderLabel('funksoul')
    i = TreeWidgetItemChild()    
    tree.addTopLevelItem(i)    
    print 'Before clearing'
    #tree.clear()                  # Didn't call destructor (__del__)
    #tree.removeItemWidget (i, 0)  # Didn't call destructor
    #i.__del__()                   # Called destructor but it's called again afterward
    del i                          # Didn't call destructor
    time.sleep(1)
    print 'After clearing'

if __name__ == '__main__':
    app = QApplication(sys.argv)
    test_QTree_clear_children()

Printed as:

TreeWidgetItemChild init
Before clearing
After clearing
TreeWidgetItemChild del

Looks to me TreeWidgetItemChild gets deleted upon the termination of process, not by any of my deletion actions.

Answer 1

Python is different from C++ in the sense of memory management/deleting objects. Python has a garbage collector (GC) that manages destroying of the objects automatically. That occurs when the reference count of an object reaches zero.

del i only means 'decrement the reference count by one'. It never results in a direct call to __del__. __del__ of an object is only called when reference count reaches to zero and is about to be garbage collected. (Although this is true for CPython, it's not guaranteed for every implementation. It depends on the GC implementation. So you should not rely on __del__ at all)

Keeping story short, the call time of __del__ is ambiguous. You should never call __del__ (or any other __foo__ special methods) directly. In fact, for the reasons above you should rather avoid the use of __del__ at all (usually).

Apart from that, there is another issue.

tree.removeItemWidget(i, 0)

This does not remove an item from QTreeWidget. As the name suggests, it removes a widget from an item, not the QTreeWidgetItem. It's counterpart to the setItemWidget method, not the addTopLevelItem method.

If you need to remove a specific item from the tree, you should use takeTopLevelItem.

tree.takeTopLevelItem(tree.indexOfTopLevelItem(i))

tree.clear() is fine. It will remove every top level item from the tree.