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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