[New-bugs-announce] [issue45210] tp_dealloc docs should mention error indicator may be set

Edward Yang report at bugs.python.org
Wed Sep 15 12:47:23 EDT 2021

New submission from Edward Yang <ezyang at mit.edu>:

The fact that the error indicator may be set during tp_dealloc is somewhat well known (https://github.com/posborne/dbus-python/blob/fef4bccfc535c6c2819e3f15384600d7bc198bc5/_dbus_bindings/conn.c#L387) but it's not documented in the official manual. We should document it.

A simple suggested patch:

diff --git a/Doc/c-api/typeobj.rst b/Doc/c-api/typeobj.rst
index b17fb22b69..e7c9b13646 100644
--- a/Doc/c-api/typeobj.rst
+++ b/Doc/c-api/typeobj.rst
@@ -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
+   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):
+   .. code-block:: c
+     static void foo_dealloc(foo_object *self) {
+         PyObject *et, *ev, *etb;
+         PyErr_Fetch(&et, &ev, &etb);
+         ...
+         PyErr_Restore(et, ev, etb);
+     }
    Finally, if the type is heap allocated (:const:`Py_TPFLAGS_HEAPTYPE`), the
    deallocator should decrement the reference count for its type object after
    calling the type deallocator. In order to avoid dangling pointers, the

assignee: docs at python
components: Documentation
messages: 401854
nosy: docs at python, ezyang
priority: normal
severity: normal
status: open
title: tp_dealloc docs should mention error indicator may be set
type: enhancement
versions: Python 3.11

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