[3.14] gh-146056: Fix list.__repr__() for lists containing NULLs (GH-146129)

(cherry picked from commit 0f2246b)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>

Author: serhiy-storchaka

Doc/c-api/file.rst

@@ -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)
 

Doc/c-api/object.rst

@@ -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.
@@ -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)
 
 
@@ -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.
@@ -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)
 

Doc/c-api/unicode.rst

@@ -1874,6 +1874,8 @@ object.
 
    Call :c:func:`PyObject_Repr` on *obj* and write the output into *writer*.
 
+   *obj* should not be ``NULL``.
+
    On success, return ``0``.
    On error, set an exception, leave the writer unchanged, and return ``-1``.
 

Include/cpython/unicodeobject.h

@@ -497,6 +497,14 @@ PyAPI_FUNC(int) PyUnicodeWriter_WriteStr(
 PyAPI_FUNC(int) PyUnicodeWriter_WriteRepr(
     PyUnicodeWriter *writer,
     PyObject *obj);
+
+PyAPI_FUNC(int) _PyUnicodeWriter_WriteReprTrue(
+    PyUnicodeWriter *writer,
+    PyObject *obj);
+#if defined(Py_BUILD_CORE) || defined(Py_BUILD_CORE_MODULE)
+#define PyUnicodeWriter_WriteRepr _PyUnicodeWriter_WriteReprTrue
+#endif
+
 PyAPI_FUNC(int) PyUnicodeWriter_WriteSubstring(
     PyUnicodeWriter *writer,
     PyObject *str,

Lib/test/test_capi/test_list.py

@@ -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()

Lib/test/test_capi/test_tuple.py

@@ -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()

Lib/test/test_capi/test_unicode.py

@@ -1765,6 +1765,14 @@ 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)
+        # CRASHES writer.write_repr(NULL)
+        writer.write_repr_true(NULL)
+        self.assertEqual(writer.finish(),
+                         "var=<NULL>")
+
     def test_utf8(self):
         writer = self.create_writer(0)
         writer.write_utf8(b"ascii", -1)

Misc/NEWS.d/next/C_API/2026-03-18-20-18-59.gh-issue-146056.nnZIgp.rst

@@ -0,0 +1 @@
+:c:func:`PyUnicodeWriter_WriteRepr` now supports ``NULL`` argument.

Misc/NEWS.d/next/Core_and_Builtins/2026-03-18-18-52-00.gh-issue-146056.r1tVSo.rst

@@ -0,0 +1 @@
+Fix :meth:`!list.__repr__` for lists containing ``NULL``\ s.

Modules/_testcapi/unicode.c

@@ -443,6 +443,7 @@ writer_write_repr(PyObject *self_raw, PyObject *args)
     if (!PyArg_ParseTuple(args, "O", &obj)) {
         return NULL;
     }
+    NULLABLE(obj);
 
     if (PyUnicodeWriter_WriteRepr(self->writer, obj) < 0) {
         return NULL;
@@ -451,6 +452,23 @@ writer_write_repr(PyObject *self_raw, PyObject *args)
 }
 
 
+static PyObject*
+writer_write_repr_true(PyObject *self_raw, PyObject *obj)
+{
+    WriterObject *self = (WriterObject *)self_raw;
+    if (writer_check(self) < 0) {
+        return NULL;
+    }
+
+    NULLABLE(obj);
+
+    if (_PyUnicodeWriter_WriteReprTrue(self->writer, obj) < 0) {
+        return NULL;
+    }
+    Py_RETURN_NONE;
+}
+
+
 static PyObject*
 writer_write_substring(PyObject *self_raw, PyObject *args)
 {
@@ -539,6 +557,7 @@ static PyMethodDef writer_methods[] = {
     {"write_ucs4", _PyCFunction_CAST(writer_write_ucs4), METH_VARARGS},
     {"write_str", _PyCFunction_CAST(writer_write_str), METH_VARARGS},
     {"write_repr", _PyCFunction_CAST(writer_write_repr), METH_VARARGS},
+    {"write_repr_true", _PyCFunction_CAST(writer_write_repr_true), METH_O},
     {"write_substring", _PyCFunction_CAST(writer_write_substring), METH_VARARGS},
     {"decodeutf8stateful", _PyCFunction_CAST(writer_decodeutf8stateful), METH_VARARGS},
     {"get_pointer", _PyCFunction_CAST(writer_get_pointer), METH_VARARGS},