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[3.14] gh-146056: Fix repr() for lists and tuples containing NULLs (GH-146129) #146155

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Merged
serhiy-storchaka merged 4 commits into from
Mar 22, 2026
Merged

[3.14] gh-146056: Fix repr() for lists and tuples containing NULLs (GH-146129) #146155

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3af9bbb
[3.14] gh-146056: Fix list.__repr__() for lists containing NULLs (GH-…
serhiy-storchaka Mar 19, 2026
b6751c1
Merge branch '3.14' into backport-0f2246b-3.14
serhiy-storchaka Mar 20, 2026
c8b634e
Add versionchanged.
serhiy-storchaka Mar 20, 2026
f36acab
Update NEWS.
serhiy-storchaka Mar 20, 2026
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7 changes: 5 additions & 2 deletions 7 Doc/c-api/file.rst
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Expand Up @@ -123,9 +123,12 @@ the :mod:`io` APIs instead.

Write object *obj* to file object *p*. The only supported flag for *flags* is
:c:macro:`Py_PRINT_RAW`; if given, the :func:`str` of the object is written
instead of the :func:`repr`. Return ``0`` on success or ``-1`` on failure; the
appropriate exception will be set.
instead of the :func:`repr`.

If *obj* is ``NULL``, write the string ``"<NULL>"``.

Return ``0`` on success or ``-1`` on failure; the
appropriate exception will be set.

.. c:function:: int PyFile_WriteString(const char *s, PyObject *p)

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8 changes: 8 additions & 0 deletions 8 Doc/c-api/object.rst
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Expand Up @@ -319,6 +319,8 @@ Object Protocol
representation on success, ``NULL`` on failure. This is the equivalent of the
Python expression ``repr(o)``. Called by the :func:`repr` built-in function.

If argument is ``NULL``, return the string ``'<NULL>'``.

.. versionchanged:: 3.4
This function now includes a debug assertion to help ensure that it
does not silently discard an active exception.
Expand All @@ -333,6 +335,8 @@ Object Protocol
a string similar to that returned by :c:func:`PyObject_Repr` in Python 2.
Called by the :func:`ascii` built-in function.

If argument is ``NULL``, return the string ``'<NULL>'``.

.. index:: string; PyObject_Str (C function)


Expand All @@ -343,6 +347,8 @@ Object Protocol
Python expression ``str(o)``. Called by the :func:`str` built-in function
and, therefore, by the :func:`print` function.

If argument is ``NULL``, return the string ``'<NULL>'``.

.. versionchanged:: 3.4
This function now includes a debug assertion to help ensure that it
does not silently discard an active exception.
Expand All @@ -358,6 +364,8 @@ Object Protocol
a TypeError is raised when *o* is an integer instead of a zero-initialized
bytes object.

If argument is ``NULL``, return the :class:`bytes` object ``b'<NULL>'``.


.. c:function:: int PyObject_IsSubclass(PyObject *derived, PyObject *cls)

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8 changes: 6 additions & 2 deletions 8 Doc/c-api/unicode.rst
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Expand Up @@ -1842,8 +1842,6 @@ object.
On success, return ``0``.
On error, set an exception, leave the writer unchanged, and return ``-1``.

.. versionadded:: 3.14

.. c:function:: int PyUnicodeWriter_WriteWideChar(PyUnicodeWriter *writer, const wchar_t *str, Py_ssize_t size)

Write the wide string *str* into *writer*.
Expand Down Expand Up @@ -1874,9 +1872,15 @@ object.

Call :c:func:`PyObject_Repr` on *obj* and write the output into *writer*.

If *obj* is ``NULL``, write the string ``"<NULL>"`` into *writer*.

@vstinner vstinner Mar 19, 2026

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I would prefer to not change PyUnicodeWriter_WriteRepr() in a Python 3.14.x bugfix release. Can you leave it unchanged? Instead, modify list_repr() and tuple_repr() to write <NULL> string explicitly.

@serhiy-storchaka serhiy-storchaka Mar 20, 2026

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We need to modify not only list_repr() and tuple_repr(), but all code that uses PyUnicodeWriter_WriteRepr(). And what about the user code? We leave it with a mine which explodes in rare and most likely not tested case.

@vstinner vstinner Mar 20, 2026

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In the Python code base, if I read the code correctly, only tuple_repr() can call PyUnicodeWriter_WriteRepr() with NULL. For example, list_repr() cannot pass NULL, because item = Py_NewRef(v->ob_item[i]) crash if the item is NULL.

And what about the user code? We leave it with a mine which explodes in rare and most likely not tested case.

My concern is that if you develop on (the future) Python 3.14.4 with PyUnicodeWriter_WriteRepr() which accepts NULL, you can get crashes if an user uses Python 3.14.0 which doesn't accept NULL. I don't think that it's a good idea to change the API in a 3.14.x bugfix release.

@serhiy-storchaka serhiy-storchaka Mar 20, 2026

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Python 3.14.0 is broken. It crashes when you call repr() for uninitialized list or tuple, and it can crash in other cases. Does it mean that we should not backport the bugfix? I think that it is the nature of bugfix releases that they make the code which failed in earlier releases to pass.

The documentation says that PyUnicodeWriter_WriteRepr() calls PyObject_Repr() on obj and writes the output into writer. PyObject_Repr() works with NULL.

@vstinner vstinner Mar 20, 2026

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If you insist to change PyUnicodeWriter_WriteRepr() API, please add a versionchanged markup to document that the API changed.

@serhiy-storchaka serhiy-storchaka Mar 20, 2026

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If we don't fix it in 3.14, we will need a versionchanged for 3.15. But we rarely add versionchanged for bugfixes, especially if the feature was added in the same version.

@vstinner vstinner Mar 20, 2026

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I'm suggesting to add a versionchanged markup in the 3.14 change to say that the API changed in Python 3.14.x.

@serhiy-storchaka serhiy-storchaka Mar 20, 2026

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Done. Created also a PR for adding in in main.


On success, return ``0``.
On error, set an exception, leave the writer unchanged, and return ``-1``.

.. versionchanged:: 3.14.4

Added support for ``NULL``.

.. c:function:: int PyUnicodeWriter_WriteSubstring(PyUnicodeWriter *writer, PyObject *str, Py_ssize_t start, Py_ssize_t end)

Write the substring ``str[start:end]`` into *writer*.
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4 changes: 4 additions & 0 deletions 4 Lib/test/test_capi/test_list.py
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Expand Up @@ -350,6 +350,10 @@ def test_list_extend(self):
# CRASHES list_extend(NULL, [])
# CRASHES list_extend([], NULL)

def test_uninitialized_list_repr(self):
lst = _testlimitedcapi.list_new(3)
self.assertEqual(repr(lst), '[<NULL>, <NULL>, <NULL>]')


if __name__ == "__main__":
unittest.main()
4 changes: 4 additions & 0 deletions 4 Lib/test/test_capi/test_tuple.py
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Expand Up @@ -292,6 +292,10 @@ def my_iter():
self.assertEqual(tuple(my_iter()), (TAG, *range(10)))
self.assertEqual(tuples, [])

def test_uninitialized_tuple_repr(self):
tup = _testlimitedcapi.tuple_new(3)
self.assertEqual(repr(tup), '(<NULL>, <NULL>, <NULL>)')


if __name__ == "__main__":
unittest.main()
7 changes: 7 additions & 0 deletions 7 Lib/test/test_capi/test_unicode.py
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Expand Up @@ -1767,6 +1767,13 @@ def test_basic(self):
self.assertEqual(writer.finish(),
"var=long value 'repr'")

def test_repr_null(self):
writer = self.create_writer(0)
writer.write_utf8(b'var=', -1)
writer.write_repr(NULL)
self.assertEqual(writer.finish(),
"var=<NULL>")

def test_write_char(self):
writer = self.create_writer(0)
writer.write_char(0)
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1 change: 1 addition & 0 deletions 1 Misc/NEWS.d/next/C_API/2026-03-18-20-18-59.gh-issue-146056.nnZIgp.rst
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@@ -0,0 +1 @@
:c:func:`PyUnicodeWriter_WriteRepr` now supports ``NULL`` argument.
1 change: 1 addition & 0 deletions 1 Misc/NEWS.d/next/Core_and_Builtins/2026-03-18-18-52-00.gh-issue-146056.r1tVSo.rst
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@@ -0,0 +1 @@
Fix :func:`repr` for lists and tuples containing ``NULL``\ s.
2 changes: 1 addition & 1 deletion 2 Objects/listobject.c
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Expand Up @@ -595,7 +595,7 @@ list_repr_impl(PyListObject *v)
so must refetch the list size on each iteration. */
for (Py_ssize_t i = 0; i < Py_SIZE(v); ++i) {
/* Hold a strong reference since repr(item) can mutate the list */
item = Py_NewRef(v->ob_item[i]);
item = Py_XNewRef(v->ob_item[i]);

if (i > 0) {
if (PyUnicodeWriter_WriteChar(writer, ',') < 0) {
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4 changes: 4 additions & 0 deletions 4 Objects/unicodeobject.c
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Expand Up @@ -13976,6 +13976,10 @@ PyUnicodeWriter_WriteStr(PyUnicodeWriter *writer, PyObject *obj)
int
PyUnicodeWriter_WriteRepr(PyUnicodeWriter *writer, PyObject *obj)
{
if (obj == NULL) {
return _PyUnicodeWriter_WriteASCIIString((_PyUnicodeWriter*)writer, "<NULL>", 6);
}

if (Py_TYPE(obj) == &PyLong_Type) {
return _PyLong_FormatWriter((_PyUnicodeWriter*)writer, obj, 10, 0);
}
Expand Down
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