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Merged
merged 9 commits into from
Jun 9, 2025
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gh-89373: Document that error indicator may be set in tp_dealloc #28358

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6107190
Document that error indicator may be set in tp_dealloc
ezyang Sep 15, 2021
3d7a842
add blurb
ezyang Sep 15, 2021
ef342c5
Merge remote-tracking branch 'origin/main' into tp-dealloc-error-indi…
ezyang Apr 3, 2022
f068c4e
Update typeobj.rst
ezyang Apr 6, 2022
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Merge remote-tracking branch 'origin/main' into tp-dealloc-error-indi…
ezyang Jun 6, 2025
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Merge remote-tracking branch 'origin' into tp-dealloc-error-indicator
ezyang Jun 6, 2025
6c71885
Don't use deprecated PyErr_Fetch
ezyang Jun 6, 2025
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Update Doc/c-api/typeobj.rst
ezyang Jun 6, 2025
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Update Doc/c-api/typeobj.rst
vstinner Jun 9, 2025
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14 changes: 14 additions & 0 deletions Doc/c-api/typeobj.rst
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Expand Up @@ -668,6 +668,20 @@ and :c:type:`PyType_Type` effectively act as defaults.)
:c:func:`PyObject_GC_Del` if the instance was allocated using
:c:func:`PyObject_GC_New` or :c:func:`PyObject_GC_NewVar`.

If you may call functions that may set the error indicator, you must
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As I understand it, very little of the C API is safe to call with an active error set. So maybe this should be a stronger message?

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IIRC, the intent here was not to discuss C API functions, but if you were calling into userland (which happens if you've got some complicated cleanup functions; at least that's what bit us here.)

use :c:func:`PyErr_Fetch` and :c:func:`PyErr_Restore` to ensure you
don't clobber a preexisting error indicator (the deallocation could
have occurred while processing a different error):
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I would add that the function must not raise an exception. It can use PyErr_WriteUnraisable() to log (and clear) an "unraisable" exception.

By the way, I'm surprised that _Py_Dealloc() doesn't ensure in debug mode (Py_DEBUG) that tp_dealloc does not raise a new exception. See also _Py_CheckSlotResult() and _Py_CheckFunctionResult().

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done!


.. code-block:: c

static void foo_dealloc(foo_object *self) {
PyObject *et, *ev, *etb;
PyErr_Fetch(&et, &ev, &etb);
...
PyErr_Restore(et, ev, etb);
}

If the type supports garbage collection (has the :const:`Py_TPFLAGS_HAVE_GC`
flag bit set), the destructor should call :c:func:`PyObject_GC_UnTrack`
before clearing any member fields.
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Documentation/2021-09-15-13-07-25.bpo-45210.RtGk7i.rst
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@@ -0,0 +1,2 @@
Document that error indicator may be set in tp_dealloc, and how to avoid
clobbering it.