Issue #25182: The stdprinter (used as sys.stderr before the io module is

imported at startup) now uses the backslashreplace error handler.

Misc/NEWS

@@ -10,6 +10,9 @@ Release date: tba
 Core and Builtins
 -----------------
 
+- Issue #25182: The stdprinter (used as sys.stderr before the io module is
+  imported at startup) now uses the backslashreplace error handler.
+
 - Issue #24891: Fix a race condition at Python startup if the file descriptor
   of stdin (0), stdout (1) or stderr (2) is closed while Python is creating
   sys.stdin, sys.stdout and sys.stderr objects. These attributes are now set

Objects/fileobject.c

@@ -372,8 +372,11 @@ PyFile_NewStdPrinter(int fd)
 static PyObject *
 stdprinter_write(PyStdPrinter_Object *self, PyObject *args)
 {
-    char *c;
+    PyObject *unicode;
+    PyObject *bytes = NULL;
+    char *str;
     Py_ssize_t n;
+    int _errno;
 
     if (self->fd < 0) {
         /* fd might be invalid on Windows
@@ -383,24 +386,37 @@ stdprinter_write(PyStdPrinter_Object *self, PyObject *args)
         Py_RETURN_NONE;
     }
 
-    if (!PyArg_ParseTuple(args, "s", &c)) {
+    if (!PyArg_ParseTuple(args, "U", &unicode))
         return NULL;
+
+    /* encode Unicode to UTF-8 */
+    str = PyUnicode_AsUTF8AndSize(unicode, &n);
+    if (str == NULL) {
+        PyErr_Clear();
+        bytes = _PyUnicode_AsUTF8String(unicode, "backslashreplace");
+        if (bytes == NULL)
+            return NULL;
+        if (PyBytes_AsStringAndSize(bytes, &str, &n) < 0) {
+            Py_DECREF(bytes);
+            return NULL;
+        }
     }
-    n = strlen(c);
 
     Py_BEGIN_ALLOW_THREADS
     errno = 0;
 #ifdef MS_WINDOWS
     if (n > INT_MAX)
         n = INT_MAX;
-    n = write(self->fd, c, (int)n);
+    n = write(self->fd, str, (int)n);
 #else
-    n = write(self->fd, c, n);
+    n = write(self->fd, str, n);
 #endif
+    _errno = errno;
     Py_END_ALLOW_THREADS
+    Py_XDECREF(bytes);
 
     if (n < 0) {
-        if (errno == EAGAIN)
+        if (_errno == EAGAIN)
             Py_RETURN_NONE;
         PyErr_SetFromErrno(PyExc_IOError);
         return NULL;