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gh-134786: Py_TPFLAGS_MANAGED_WEAKREF implies Py_TPFLAGS_HAVE_GC too and force checking of its presence #135863

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gh-134786: Py_TPFLAGS_MANAGED_WEAKREF implies Py_TPFLAGS_HAVE_GC too and force checking of its presence #135863

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Py_TPFLAGS_MANAGED_WEAKREF implies Py_TPFLAGS_HAVE_GC too and force c…
sergey-miryanov Jun 23, 2025
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sergey-miryanov Jun 24, 2025
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sergey-miryanov Jun 24, 2025
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2 changes: 2 additions & 0 deletions Doc/c-api/typeobj.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -1279,6 +1279,8 @@ and :c:data:`PyType_Type` effectively act as defaults.)
This bit indicates that instances of the class should be weakly
referenceable.

If this flag is set, :c:macro:`Py_TPFLAGS_HAVE_GC` should also be set.

.. versionadded:: 3.12

**Inheritance:**
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2 changes: 2 additions & 0 deletions Doc/extending/newtypes.rst
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Expand Up @@ -560,6 +560,8 @@ For an object to be weakly referenceable, the extension type must set the
field. The legacy :c:member:`~PyTypeObject.tp_weaklistoffset` field should
be left as zero.

If this flag is set, :c:macro:`Py_TPFLAGS_HAVE_GC` should also be set.

Concretely, here is how the statically declared type object would look::

static PyTypeObject TrivialType = {
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8 changes: 8 additions & 0 deletions Doc/whatsnew/3.15.rst
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Expand Up @@ -310,6 +310,14 @@ New features
(Contributed by Petr Viktorin in :gh:`131510`)


Limited C API changes
---------------------

* If the :c:macro:`Py_TPFLAGS_MANAGED_DICT` and :c:macro:`Py_TPFLAGS_MANAGED_WEAKREF`
flags are set then :c:macro:`Py_TPFLAGS_HAVE_GC` should be set too.
(Contributed by Sergey Miryanov in :gh:`134786`)


Porting to Python 3.15
----------------------

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2 changes: 1 addition & 1 deletion Include/object.h
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Expand Up @@ -525,7 +525,7 @@ given type object has a specified feature.
#define Py_TPFLAGS_INLINE_VALUES (1 << 2)

/* Placement of weakref pointers are managed by the VM, not by the type.
* The VM will automatically set tp_weaklistoffset.
* The VM will automatically set tp_weaklistoffset. Implies Py_TPFLAGS_HAVE_GC.
*/
#define Py_TPFLAGS_MANAGED_WEAKREF (1 << 3)

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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-06-24-13-12-58.gh-issue-134786.MF0VVk.rst
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@@ -0,0 +1,3 @@
Force to use :c:macro:`Py_TPFLAGS_HAVE_GC` if
:c:macro:`Py_TPFLAGS_MANAGED_DICT` or :c:macro:`Py_TPFLAGS_MANAGED_WEAKREF`
used.
14 changes: 14 additions & 0 deletions Objects/typeobject.c
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Expand Up @@ -8804,6 +8804,13 @@ type_ready_preheader(PyTypeObject *type)
type->tp_name);
return -1;
}
if (!(type->tp_flags & Py_TPFLAGS_HAVE_GC)) {
PyErr_Format(PyExc_SystemError,
"type %s has the Py_TPFLAGS_MANAGED_DICT flag "
"but not Py_TPFLAGS_HAVE_GC flag",
type->tp_name);
return -1;
}
type->tp_dictoffset = -1;
}
if (type->tp_flags & Py_TPFLAGS_MANAGED_WEAKREF) {
Expand All @@ -8816,6 +8823,13 @@ type_ready_preheader(PyTypeObject *type)
type->tp_name);
return -1;
}
if (!(type->tp_flags & Py_TPFLAGS_HAVE_GC)) {
PyErr_Format(PyExc_SystemError,
"type %s has the Py_TPFLAGS_MANAGED_WEAKREF flag "
"but not Py_TPFLAGS_HAVE_GC flag",
type->tp_name);
return -1;
}
type->tp_weaklistoffset = MANAGED_WEAKREF_OFFSET;
}
return 0;
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