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

imported at startup) now uses the backslashreplace error handler.

diff --cc Misc/NEWS
index 487eea184335648bedd1f854ceabdc1ffa5a4e55,c3068c4260b5287fd02945ece95f4e622a1e37b2..55f4e40d6c456fff48fbd909bdba5e46f24b304f
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -11,12 -10,73 +11,15 @@@ Release date: TB
  Core and Builtins
  -----------------
  
 -- 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
 -  to None if the creation of the object failed, instead of raising an OSError
 -  exception. Initial patch written by Marco Paolini.
 -
 -- Issue #21167: NAN operations are now handled correctly when python is
 -  compiled with ICC even if -fp-model strict is not specified.
 -
 -- Issue #4395: Better testing and documentation of binary operators.
 -  Patch by Martin Panter.
 -
 -- Issue #24467: Fixed possible buffer over-read in bytearray. The bytearray
 -  object now always allocates place for trailing null byte and it's buffer now
 -  is always null-terminated.
 -
 -- Issue #24115: Update uses of PyObject_IsTrue(), PyObject_Not(),
 -  PyObject_IsInstance(), PyObject_RichCompareBool() and _PyDict_Contains()
 -  to check for and handle errors correctly.
 -
 -- Issue #24257: Fixed system error in the comparison of faked
 -  types.SimpleNamespace.
 -
 -- Issue #22939: Fixed integer overflow in iterator object.  Patch by
 -  Clement Rouault.
 -
 -- Issue #23985: Fix a possible buffer overrun when deleting a slice from
 -  the front of a bytearray and then appending some other bytes data.
 -
 -- Issue #24102: Fixed exception type checking in standard error handlers.
 -
 -- Issue #23757: PySequence_Tuple() incorrectly called the concrete list API
 -  when the data was a list subclass.
 -
 -- Issue #24407: Fix crash when dict is mutated while being updated.
 -
 -- Issue #24096: Make warnings.warn_explicit more robust against mutation of the
 -  warnings.filters list.
 -
 -- Issue #23996: Avoid a crash when a delegated generator raises an
 -  unnormalized StopIteration exception.  Patch by Stefan Behnel.
 -
 -- Issue #24022: Fix tokenizer crash when processing undecodable source code.
 -
 -- Issue #23309: Avoid a deadlock at shutdown if a daemon thread is aborted
 -  while it is holding a lock to a buffered I/O object, and the main thread
 -  tries to use the same I/O object (typically stdout or stderr).  A fatal
 -  error is emitted instead.
 -
 -- Issue #22977: Fixed formatting Windows error messages on Wine.
 -  Patch by Martin Panter.
 -
 -- Issue #23803: Fixed str.partition() and str.rpartition() when a separator
 -  is wider then partitioned string.
 -
 -- Issue #23192: Fixed generator lambdas.  Patch by Bruno Cauet.
 -
 -- Issue #23629: Fix the default __sizeof__ implementation for variable-sized
 -  objects.
 -
 -- Issue #24044: Fix possible null pointer dereference in list.sort in out of
 -  memory conditions.
+ - Issue #25182: The stdprinter (used as sys.stderr before the io module is
+   imported at startup) now uses the backslashreplace error handler.
+ 
 +- Issue #25131: Make the line number and column offset of set/dict literals and
 +  comprehensions correspond to the opening brace.
  
 -- Issue #21354: PyCFunction_New function is exposed by python DLL again.
 +- Issue #25150: Hide the private _Py_atomic_xxx symbols from the public
 +  Python.h header to fix a compilation error with OpenMP. PyThreadState_GET()
 +  becomes an alias to PyThreadState_Get() to avoid ABI incompatibilies.
  
  Library
  -------

diff --cc Objects/fileobject.c
index 1b184100d0b9fd6cf465501e3bd40e065abfa055,403d718dfe5489894c5dad08e3091c113265c195..a836cb38155d76755d736bd86009236b481acbf2
--- 1/Objects/fileobject.c
--- 2/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@@ -383,16 -386,39 +386,30 @@@ stdprinter_write(PyStdPrinter_Object *s
          Py_RETURN_NONE;
      }
  
-     /* encode Unicode to UTF-8 */
-     if (!PyArg_ParseTuple(args, "s", &str))
+     if (!PyArg_ParseTuple(args, "U", &unicode))
          return NULL;
  
-     n = _Py_write(self->fd, str, strlen(str));
+     /* 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;
+         }
+     }
+ 
 -    Py_BEGIN_ALLOW_THREADS
 -    errno = 0;
 -#ifdef MS_WINDOWS
 -    if (n > INT_MAX)
 -        n = INT_MAX;
 -    n = write(self->fd, str, (int)n);
 -#else
 -    n = write(self->fd, str, n);
 -#endif
++    n = _Py_write(self->fd, str, n);
+     _errno = errno;
 -    Py_END_ALLOW_THREADS
+     Py_XDECREF(bytes);
 -
 -    if (n < 0) {
 -        if (_errno == EAGAIN)
 +    if (n == -1) {
-         if (errno == EAGAIN) {
++        if (_errno == EAGAIN) {
 +            PyErr_Clear();
              Py_RETURN_NONE;
 -        PyErr_SetFromErrno(PyExc_IOError);
 +        }
          return NULL;
      }