python
diff --git a/‎Include/cpython/unicodeobject.h‎
Lines changed: 6 additions & 0 deletions b/‎Include/cpython/unicodeobject.h‎
Lines changed: 6 additions & 0 deletions
diff --git a/‎Modules/_testcapi/unicode.c‎
Lines changed: 34 additions & 0 deletions b/‎Modules/_testcapi/unicode.c‎
Lines changed: 34 additions & 0 deletions
diff --git a/‎Objects/unicodeobject.c‎
Lines changed: 22 additions & 0 deletions b/‎Objects/unicodeobject.c‎
Lines changed: 22 additions & 0 deletions
@@ -459,6 +459,12 @@ PyAPI_FUNC(int) PyUnicodeWriter_WriteUTF8(
     PyUnicodeWriter *writer,
     const char *str,
     Py_ssize_t size);
+PyAPI_FUNC(int) PyUnicodeWriter_DecodeUTF8Stateful(
+    PyUnicodeWriter *writer,
+    const char *string,         /* UTF-8 encoded string */
+    Py_ssize_t length,          /* size of string */
+    const char *errors,         /* error handling */
+    Py_ssize_t *consumed);      /* bytes consumed */
 PyAPI_FUNC(int) PyUnicodeWriter_WriteWideChar(
     PyUnicodeWriter *writer,
     wchar_t *str,
 
@@ -374,6 +374,39 @@ test_unicodewriter_recover_error(PyObject *self, PyObject *Py_UNUSED(args))
 }
 
 
+static PyObject *
+test_unicodewriter_decode_utf8(PyObject *self, PyObject *Py_UNUSED(args))
+{
+    // test PyUnicodeWriter_DecodeUTF8Stateful()
+    PyUnicodeWriter *writer = PyUnicodeWriter_Create(0);
+    if (writer == NULL) {
+        return NULL;
+    }
+    if (PyUnicodeWriter_DecodeUTF8Stateful(writer, "ign\xFFore", -1, "ignore", NULL) < 0) {
+        goto error;
+    }
+    if (PyUnicodeWriter_WriteChar(writer, '-') < 0) {
+        goto error;
+    }
+    if (PyUnicodeWriter_DecodeUTF8Stateful(writer, "replace\xFF", -1, "replace", NULL) < 0) {
+        goto error;
+    }
+
+    PyObject *result = PyUnicodeWriter_Finish(writer);
+    if (result == NULL) {
+        return NULL;
+    }
+    assert(PyUnicode_EqualToUTF8(result, "ignore-replace\xef\xbf\xbd"));
+    Py_DECREF(result);
+
+    Py_RETURN_NONE;
+
+error:
+    PyUnicodeWriter_Discard(writer);
+    return NULL;
+}
+
+
 static PyObject *
 test_unicodewriter_format(PyObject *self, PyObject *Py_UNUSED(args))
 {
@@ -484,6 +517,7 @@ static PyMethodDef TestMethods[] = {
     {"test_unicodewriter_utf8",  test_unicodewriter_utf8,        METH_NOARGS},
     {"test_unicodewriter_invalid_utf8", test_unicodewriter_invalid_utf8, METH_NOARGS},
     {"test_unicodewriter_recover_error", test_unicodewriter_recover_error, METH_NOARGS},
+    {"test_unicodewriter_decode_utf8", test_unicodewriter_decode_utf8, METH_NOARGS},
     {"test_unicodewriter_format", test_unicodewriter_format,     METH_NOARGS},
     {"test_unicodewriter_format_recover_error", test_unicodewriter_format_recover_error, METH_NOARGS},
     {"test_unicodewriter_widechar", test_unicodewriter_widechar, METH_NOARGS},
 
@@ -13501,6 +13501,28 @@ PyUnicodeWriter_WriteUTF8(PyUnicodeWriter *writer,
 }
 
 
+int
+PyUnicodeWriter_DecodeUTF8Stateful(PyUnicodeWriter *writer,
+                                   const char *string,
+                                   Py_ssize_t length,
+                                   const char *errors,
+                                   Py_ssize_t *consumed)
+{
+    if (length < 0) {
+        length = strlen(string);
+    }
+
+    _PyUnicodeWriter *_writer = (_PyUnicodeWriter*)writer;
+    Py_ssize_t old_pos = _writer->pos;
+    int res = unicode_decode_utf8_writer(_writer, string, length,
+                                         _Py_ERROR_UNKNOWN, errors, consumed);
+    if (res < 0) {
+        _writer->pos = old_pos;
+    }
+    return res;
+}
+
+
 int
 PyUnicodeWriter_WriteWideChar(PyUnicodeWriter *writer,
                               wchar_t *str,