Mar 22, 2026 · Mar 19, 2026 · Mar 20, 2026 · Mar 20, 2026 · Mar 20, 2026 · Mar 19, 2026
diff --git a/Doc/c-api/file.rst b/Doc/c-api/file.rst

   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)

diff --git a/Doc/c-api/object.rst b/Doc/c-api/object.rst
   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.
   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)


   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.
   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)

diff --git a/Doc/c-api/unicode.rst b/Doc/c-api/unicode.rst
   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*.

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

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

   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*.
diff --git a/Lib/test/test_capi/test_list.py b/Lib/test/test_capi/test_list.py
 # CRASHES list_extend(NULL, [])
 # CRASHES list_extend([], NULL)

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


 if__name__=="__main__":
 unittest.main()
diff --git a/Lib/test/test_capi/test_tuple.py b/Lib/test/test_capi/test_tuple.py
        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()
diff --git a/Lib/test/test_capi/test_unicode.py b/Lib/test/test_capi/test_unicode.py
        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)
diff --git a/Misc/NEWS.d/next/C_API/2026-03-18-20-18-59.gh-issue-146056.nnZIgp.rst b/Misc/NEWS.d/next/C_API/2026-03-18-20-18-59.gh-issue-146056.nnZIgp.rst
 :c:func:`PyUnicodeWriter_WriteRepr` now supports ``NULL`` argument.
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-03-18-18-52-00.gh-issue-146056.r1tVSo.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-18-18-52-00.gh-issue-146056.r1tVSo.rst
 Fix :func:`repr` for lists and tuples containing ``NULL``\ s.
diff --git a/Objects/listobject.c b/Objects/listobject.c
       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) {
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
 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);
    }
Original file line number	Diff line number	Diff line change
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)

Expand Down
Original file line number	Diff line number	Diff line change
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)

Expand Down
Original file line number	Diff line number	Diff line change
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 DownExpand 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.
Copy link Member vstinnerMar 19, 2026 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. 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. Copy link MemberAuthor serhiy-storchakaMar 20, 2026 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. 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. Copy link Member vstinnerMar 20, 2026 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. 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. Copy link MemberAuthor serhiy-storchakaMar 20, 2026 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. 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()` onobj and writes the output intowriter.`PyObject_Repr()` works with NULL. Copy link Member vstinnerMar 20, 2026 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. If you insist to change PyUnicodeWriter_WriteRepr() API, please add a`versionchanged` markup to document that the API changed. Copy link MemberAuthor serhiy-storchakaMar 20, 2026 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. 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. Copy link Member vstinnerMar 20, 2026 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. I'm suggesting to add a`versionchanged` markup in the 3.14 change to say that the API changed in Python 3.14.x. Copy link MemberAuthor serhiy-storchakaMar 20, 2026 Choose a reason for hiding this comment The reason will be displayed to describe this comment to others.Learn more. 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.
Expand Down
Original file line number	Diff line number	Diff line change
Expand Up		@@ -350,6 +350,10 @@ def test_list_extend(self):
		# CRASHES list_extend(NULL, [])
		# CRASHES list_extend([], NULL)

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


		if__name__=="__main__":
		unittest.main()
Original file line number	Diff line number	Diff line change
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()
Original file line number	Diff line number	Diff line change
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)
Expand Down
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		:c:func:`PyUnicodeWriter_WriteRepr` now supports ``NULL`` argument.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Fix :func:`repr` for lists and tuples containing ``NULL``\ s.
Original file line number	Diff line number	Diff line change
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) {
Expand Down
Original file line number	Diff line number	Diff line change
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