On 3/17/2017 10:54 AM, Oleg Nesterov wrote:
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.
If I understand the below, 'that persists after the function call'
should be added. Note that the function call itself 'creates a new
reference' by binding 'self' to the obj.
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;
I suspect that this was added after the doc. If git has an annotate
function, you could check.
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.
--
Terry Jan Reedy
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