Add PyUnicodeWriter_WriteUCS4() function

Author: vstinner

docs/api.rst

@@ -193,6 +193,12 @@ Python 3.14
 
    Not available on the limited C API.
 
+.. c:function:: int PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *writer, const Py_UCS4 *ucs4, Py_ssize_t size)
+
+   See `PyUnicodeWriter_WriteUCS4() documentation <https://docs.python.org/dev/c-api/unicode.html#c.PyUnicodeWriter_WriteUCS4>`__.
+
+   Not available on the limited C API.
+
 .. c:function:: int PyUnicodeWriter_WriteUTF8(PyUnicodeWriter *writer, const char *str, Py_ssize_t size)
 
    See `PyUnicodeWriter_WriteUTF8() documentation <https://docs.python.org/dev/c-api/unicode.html#c.PyUnicodeWriter_WriteUTF8>`__.
@@ -338,7 +344,6 @@ Not supported:
 * ``PyInitConfig_SetStrList()``
 * ``PyType_GetBaseByToken()``
 * ``PyUnicodeWriter_DecodeUTF8Stateful()``
-* ``PyUnicodeWriter_WriteUCS4()``
 * ``Py_InitializeFromInitConfig()``
 
 

docs/changelog.rst

@@ -1,6 +1,7 @@
 Changelog
 =========
 
+* 2026-10-10: Add ``PyUnicodeWriter_WriteUCS4()`` function.
 * 2026-10-08: Add support for the limited C API 3.2.
 * 2026-10-08: Add functions:
 

pythoncapi_compat.h

@@ -1620,6 +1620,21 @@ PyUnicodeWriter_WriteSubstring(PyUnicodeWriter *writer, PyObject *str,
                                            start, end);
 }
 
+static inline int
+PyUnicodeWriter_WriteUCS4(PyUnicodeWriter *writer,
+                          const Py_UCS4 *ucs4, Py_ssize_t size)
+{
+    PyObject *str = PyUnicode_FromKindAndData(PyUnicode_4BYTE_KIND,
+                                              ucs4, size);
+    if (str == NULL) {
+        return -1;
+    }
+
+    int res = _PyUnicodeWriter_WriteStr((_PyUnicodeWriter*)writer, str);
+    Py_DECREF(str);
+    return res;
+}
+
 static inline int
 PyUnicodeWriter_Format(PyUnicodeWriter *writer, const char *format, ...)
 {

tests/test_pythoncapi_compat_cext.c

@@ -1899,6 +1899,7 @@ test_unicodewriter(PyObject *Py_UNUSED(self), PyObject *Py_UNUSED(args))
         return NULL;
     }
     int ret;
+    const Py_UCS4 ucs4[] = {'u', 'c', 's', '4'};
 
     // test PyUnicodeWriter_WriteStr()
     PyObject *str = PyUnicode_FromString("var");
@@ -1940,6 +1941,14 @@ test_unicodewriter(PyObject *Py_UNUSED(self), PyObject *Py_UNUSED(args))
         goto error;
     }
 
+    // test PyUnicodeWriter_WriteUCS4()
+    if (PyUnicodeWriter_WriteUCS4(writer, ucs4, Py_ARRAY_LENGTH(ucs4)) < 0) {
+        goto error;
+    }
+    if (PyUnicodeWriter_WriteChar(writer, ' ') < 0) {
+        goto error;
+    }
+
     // test PyUnicodeWriter_WriteRepr()
     str = PyUnicode_FromString("repr");
     if (str == NULL) {
@@ -1958,7 +1967,7 @@ test_unicodewriter(PyObject *Py_UNUSED(self), PyObject *Py_UNUSED(args))
         if (result == NULL) {
             return NULL;
         }
-        const char *expected = "var=long non-ASCII valu\xC3\xA9 'repr'<NULL>";
+        const char *expected = "var=long non-ASCII valu\xC3\xA9 ucs4 'repr'<NULL>";
         assert(PyUnicode_EqualToUTF8(result, expected));
         Py_DECREF(result);
     }