This issue tracker has been migrated to GitHub, and is currently read-only.
For more information, see the GitHub FAQs in the Python's Developer Guide.

Author vstinner
Recipients christian.heimes, corona10, erlendaasland, kj, miss-islington, nascheme, ncoghlan, pablogsal, shihai1991, vstinner
Date 2021-05-27.16:58:39
SpamBayes Score -1.0
Marked as misclassified Yes
Message-id <>
I'm not sure that it's safe to call PyObject_ClearWeakRefs() in tp_clear function. PyObject_ClearWeakRefs() comment starts with:

"This function is called by the tp_dealloc handler to clear weak references."

For example in Modules/arraymodule.c, I don't understand well what assigns weakreflist and what is the object type. Is it a strong reference to a Python object? What is supposed to call Py_DECREF() on it?

Is it enought to call PyObject_ClearWeakRefs() in tp_dealloc?

subtype_dealloc() calls PyObject_ClearWeakRefs(self).

I wrote a short example:
import weakref
import ctypes
import sys

class A:

assert obj.__weakref__ is None

wr1 = weakref.ref(obj)
assert obj.__weakref__ is wr1
print("refcnt(wr1)", sys.getrefcount(wr1))

wr2 = weakref.ref(obj)
assert wr2 is wr1
assert obj.__weakref__ is wr1
print("refcnt(wr1)", sys.getrefcount(wr1))

_PyWeakref_GetWeakrefCount = ctypes.pythonapi._PyWeakref_GetWeakrefCount
_PyWeakref_GetWeakrefCount.argtypes = (ctypes.py_object,)
_PyWeakref_GetWeakrefCount.restype = ctypes.c_size_t
print("_PyWeakref_GetWeakrefCount:", _PyWeakref_GetWeakrefCount(wr1))

<class 'weakref'>
refcnt(wr1) 2
refcnt(wr1) 3
_PyWeakref_GetWeakrefCount: 1

In this case, wr2 is wr1, __weakreflist__ points to a weakref.ref object instance, and _PyWeakref_GetWeakrefCount() returns 1.

At the first weakref.ref() call, the reference count is 1: "wr1" variable holds this reference. I understand that obj stores a *weak* reference to the Python object "weakref.ref". So it doesn't have to DECREF anything.
Date User Action Args
2021-05-27 16:58:40vstinnersetrecipients: + vstinner, nascheme, ncoghlan, christian.heimes, corona10, pablogsal, miss-islington, shihai1991, erlendaasland, kj
2021-05-27 16:58:40vstinnersetmessageid: <>
2021-05-27 16:58:40vstinnerlinkissue42972 messages
2021-05-27 16:58:39vstinnercreate