[Python-Dev] del is not called after creating a new reference (original) (raw)

Oleg Nesterov oleg at redhat.com
Mon Mar 20 13:28:29 EDT 2017

Hello,

I already tried to ask on python-list, see https://mail.python.org/pipermail/python-list/2017-March/720037.html but it seems that this list is not for technical questions.

Let me resend my question to python-dev. Please tell me if I should not spam this list with newbiesh questions, and thanks in advance.

I started to learn python a few days ago and I am trying to understand what del() actually does. https://docs.python.org/3/reference/datamodel.html says:

object.__del__(self)
...
Note that it is possible (though not recommended!) for the __del__()
method to postpone destruction of the instance by creating a new
reference to it. It may then be called at a later time when this new
reference is deleted.

However, this trivial test-case

class C:
    def __del__(self):
        print("DEL")
        global X
        X = self
C()
print(X)
X = 0
print(X)

shows that del is called only once, it is not called again after "X = 0":

DEL
<__main__.C object at 0x7f067695f4a8>
0

(Just in case, I verified later that this object actually goes away and its memory is freed, so the problem is not that it still has a reference).

I've cloned https://github.com/python/cpython.git and everything looks clear at first glance (but let me repeat that I am very new to python):

PyObject_CallFinalizerFromDealloc() calls PyObject_CallFinalizer()
which finally calls "__del__" method in slot_tp_finalize(), then it
notices that "X = self" creates the new reference and does:

/* tp_finalize resurrected it!  Make it look like the original Py_DECREF
 * never happened.
 */
refcnt = self->ob_refcnt;
_Py_NewReference(self);
self->ob_refcnt = refcnt;

However, PyObject_CallFinalizer() also does _PyGC_SET_FINALIZED(self, 1) and that is why del is not called again after "X = 0":

/* tp_finalize should only be called once. */
if (PyType_IS_GC(tp) && _PyGC_FINALIZED(self))
    return;

The comment and the code are very explicit, so this does nt look like a bug in cpython.

Probably the docs should be fixed?

Or this code is actually wrong? The test-case works as documented if I remove _PyGC_SET_FINALIZED() in PyObject_CallFinalizer() or add another _PyGC_SET_FINALIZED(self, 0) into PyObject_CallFinalizerFromDealloc() after _Py_NewReference(self), but yes, yes, I understand that this is not correct and won't really help.

Oleg.

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