How to know if object gets deleted in Python

Question

I have an object in the heap and a reference to it. There are certain circumstances in which the object gets deleted but the reference that points to its location doesn't know that. How can I check if there is real data in the heap?

For example:

from PySide import *
a = QProgressBar()
b = QProgressBar()
self.setIndexWidget(index,a)
self.setIndexWidget(index,b)

Then the a object gets deleted but print(a) returns a valid address. However if you try a.value() - runtime error occurs (C++ object already deleted).

a is None returns False.

Answer 1

use sip module, read more about sip here

import sip

a = QProgressBar()
sip.isdeleted(a)
False

sip.delete(a)
a
<PyQt4.QtCore.QObject object at 0x017CCA98>

sip.isdeleted(a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: underlying C/C++ object has been deleted

Answer 2

For the PySide objects you'll need the shiboken module to perform object queries.

Visit the shiboken module documention:

import shiboken

print shiboken.isValid(a)

Answer 3

It is explicitly mentioned in the documentation when an object takes the responsibility for the deletion of another object. In your example, you can see this in the Qt doc :

If index widget A is replaced with index widget B, index widget A will be deleted.