From becd7a967f36248e0090010fbd04885170df3e7a Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Thu, 19 Mar 2026 10:23:01 +0200
Subject: [PATCH] gh-146143: Fix the PyUnicodeWriter_WriteUCS4() signature
 (GH-146144)

It now accepts a pointer to constant buffer of Py_UCS4.
---
 Doc/c-api/unicode.rst                                           | 2 +-
 Include/cpython/unicodeobject.h                                 | 2 +-
 .../next/C_API/2026-03-18-23-44-29.gh-issue-146143.pwIrJq.rst   | 2 ++
 Objects/unicodeobject.c                                         | 2 +-
 4 files changed, 5 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/C_API/2026-03-18-23-44-29.gh-issue-146143.pwIrJq.rst

diff --git a/Doc/c-api/unicode.rst b/Doc/c-api/unicode.rst
index 7e9346ea9f7..3a9b34b5922 100644
--- a/Doc/c-api/unicode.rst
+++ b/Doc/c-api/unicode.rst
@@ -1867,7 +1867,7 @@ object.
    On success, return ``0``.
    On error, set an exception, leave the writer unchanged, and return ``-1``.
 
-.. c:function:: int PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *writer, Py_UCS4 *str, Py_ssize_t size)
+.. c:function:: int PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *writer, const Py_UCS4 *str, Py_ssize_t size)
 
    Writer the UCS4 string *str* into *writer*.
 
diff --git a/Include/cpython/unicodeobject.h b/Include/cpython/unicodeobject.h
index 631a6570658..ea91f4158eb 100644
--- a/Include/cpython/unicodeobject.h
+++ b/Include/cpython/unicodeobject.h
@@ -496,7 +496,7 @@ PyAPI_FUNC(int) PyUnicodeWriter_WriteWideChar(
     Py_ssize_t size);
 PyAPI_FUNC(int) PyUnicodeWriter_WriteUCS4(
     PyUnicodeWriter *writer,
-    Py_UCS4 *str,
+    const Py_UCS4 *str,
     Py_ssize_t size);
 
 PyAPI_FUNC(int) PyUnicodeWriter_WriteStr(
diff --git a/Misc/NEWS.d/next/C_API/2026-03-18-23-44-29.gh-issue-146143.pwIrJq.rst b/Misc/NEWS.d/next/C_API/2026-03-18-23-44-29.gh-issue-146143.pwIrJq.rst
new file mode 100644
index 00000000000..930d90ff9a2
--- /dev/null
+++ b/Misc/NEWS.d/next/C_API/2026-03-18-23-44-29.gh-issue-146143.pwIrJq.rst
@@ -0,0 +1,2 @@
+:c:func:`PyUnicodeWriter_WriteUCS4` now accepts a pointer to a constant buffer
+of ``Py_UCS4``.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index dd2482ca653..d51a95c69a9 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -2224,7 +2224,7 @@ _PyUnicode_FromUCS4(const Py_UCS4 *u, Py_ssize_t size)
 
 int
 PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *pub_writer,
-                          Py_UCS4 *str,
+                          const Py_UCS4 *str,
                           Py_ssize_t size)
 {
     _PyUnicodeWriter *writer = (_PyUnicodeWriter*)pub_writer;