gh-86427: Fix the stdio encoding in the legacy Windows stdio mode

It was the ANSI code page instead of the encoding of the device the stream
is connected to, as in 3.7.

The stdio encoding is now left undefined in this mode and determined for
every standard stream.

Author: serhiy-storchaka

Lib/test/test_cmd_line.py

@@ -1067,6 +1067,42 @@ def test_python_legacy_windows_stdio(self):
         support.skip_on_low_desktop_heap_memory_subprocess(p.returncode)
         self.assertEqual(p.returncode, 0)
 
+    @unittest.skipUnless(support.MS_WINDOWS, 'Test only applicable on Windows')
+    def test_python_legacy_windows_stdio_encoding(self):
+        # gh-86427: In the legacy mode the encoding of a standard stream is
+        # the encoding of the console it is connected to, which can differ
+        # for input and output.
+        import ctypes
+        kernel32 = ctypes.WinDLL('kernel32', use_last_error=True)
+        try:
+            fin = open('CONIN$')
+        except OSError:
+            self.skipTest('no console')
+        # We cannot use PIPE, because the standard streams should be
+        # connected to the console.  So we use the exit code.
+        code = ("import sys; sys.exit(sys.stdin.encoding != 'cp850' or "
+                "sys.stdout.encoding != 'cp437')")
+        env = os.environ.copy()
+        env['PYTHONLEGACYWINDOWSSTDIO'] = '1'
+        env['PYTHONUTF8'] = '0'
+        env.pop('PYTHONIOENCODING', None)
+        old_cp = kernel32.GetConsoleCP()
+        old_output_cp = kernel32.GetConsoleOutputCP()
+        with fin, open('CONOUT$', 'w') as fout:
+            try:
+                if not kernel32.SetConsoleCP(850):
+                    self.skipTest('cannot set the console input code page')
+                if not kernel32.SetConsoleOutputCP(437):
+                    self.skipTest('cannot set the console output code page')
+                proc = subprocess.run([sys.executable, '-c', code], env=env,
+                                      stdin=fin, stdout=fout,
+                                      stderr=subprocess.DEVNULL)
+            finally:
+                kernel32.SetConsoleCP(old_cp)
+                kernel32.SetConsoleOutputCP(old_output_cp)
+        support.skip_on_low_desktop_heap_memory_subprocess(proc.returncode)
+        self.assertEqual(proc.returncode, 0)
+
     @unittest.skipIf("-fsanitize" in sysconfig.get_config_vars().get('PY_CFLAGS', ()),
                      "PYTHONMALLOCSTATS doesn't work with ASAN")
     def test_python_malloc_stats(self):

Misc/NEWS.d/next/Windows/2026-08-09-05-30-00.gh-issue-86427.legacystdio.rst

@@ -0,0 +1,3 @@
+Fix the encoding of the standard streams in the legacy Windows stdio mode
+(:envvar:`PYTHONLEGACYWINDOWSSTDIO`).  It is now the encoding of the device
+the stream is connected to, as in Python 3.7, not the ANSI code page.

Objects/unicodeobject.c

@@ -15209,6 +15209,10 @@ init_stdio_encoding(PyInterpreterState *interp)
 {
     /* Update the stdio encoding to the normalized Python codec name. */
     PyConfig *config = (PyConfig*)_PyInterpreterState_GetConfig(interp);
+    if (config->stdio_encoding == NULL) {
+        /* gh-86427: The encoding is determined for every stream. */
+        return _PyStatus_OK();
+    }
     if (config_get_codec_name(&config->stdio_encoding) < 0) {
         return _PyStatus_ERR("failed to get the Python codec name "
                              "of the stdio encoding");

Python/initconfig.c

@@ -196,7 +196,7 @@ static const PyConfigSpec PYCONFIG_SPEC[] = {
     SPEC(show_ref_count, BOOL, READ_ONLY, NO_SYS, NO_GLOBAL),
     SPEC(site_import, BOOL, READ_ONLY, NO_SYS, GLOBAL(&Py_NoSiteFlag, 1)),  // sys.flags.no_site
     SPEC(skip_source_first_line, BOOL, READ_ONLY, NO_SYS, NO_GLOBAL),
-    SPEC(stdio_encoding, WSTR, READ_ONLY, NO_SYS, NO_GLOBAL),
+    SPEC(stdio_encoding, WSTR_OPT, READ_ONLY, NO_SYS, NO_GLOBAL),
     SPEC(stdio_errors, WSTR, READ_ONLY, NO_SYS, NO_GLOBAL),
     SPEC(tracemalloc, UINT, READ_ONLY, NO_SYS, NO_GLOBAL),
     SPEC(use_frozen_modules, BOOL, READ_ONLY, NO_SYS, NO_GLOBAL),
@@ -1074,11 +1074,14 @@ config_check_consistency(const PyConfig *config)
     assert(config->module_search_paths_set >= 0);
     assert(config->filesystem_encoding != NULL);
     assert(config->filesystem_errors != NULL);
-    assert(config->stdio_encoding != NULL);
-    assert(config->stdio_errors != NULL);
 #ifdef MS_WINDOWS
+    /* stdio_encoding can be NULL in the legacy Windows stdio mode. */
+    assert(config->stdio_encoding != NULL || config->legacy_windows_stdio);
     assert(config->legacy_windows_stdio >= 0);
+#else
+    assert(config->stdio_encoding != NULL);
 #endif
+    assert(config->stdio_errors != NULL);
     /* -c and -m options are exclusive */
     assert(!(config->run_command != NULL && config->run_module != NULL));
     assert(config->check_hash_pycs_mode != NULL);
@@ -2709,7 +2712,12 @@ config_init_stdio_encoding(PyConfig *config,
     }
 
     /* Choose the default error handler based on the current locale. */
-    if (config->stdio_encoding == NULL) {
+    if (config->stdio_encoding == NULL
+#ifdef MS_WINDOWS
+        /* gh-86427: it is determined for each stream. */
+        && !config->legacy_windows_stdio
+#endif
+       ) {
         status = config_get_locale_encoding(config, preconfig,
                                             &config->stdio_encoding);
         if (_PyStatus_EXCEPTION(status)) {

Python/pylifecycle.c

@@ -3070,7 +3070,18 @@ create_stdio(const PyConfig *config, PyObject* io,
     newline = "\n";
 #endif
 
-    PyObject *encoding_str = PyUnicode_FromWideChar(encoding, -1);
+    PyObject *encoding_str;
+    if (encoding != NULL) {
+        encoding_str = PyUnicode_FromWideChar(encoding, -1);
+    }
+    else {
+        /* gh-86427: use the encoding of the device. */
+        encoding_str = _Py_device_encoding(fd);
+        if (encoding_str == Py_None) {
+            Py_DECREF(encoding_str);
+            encoding_str = _Py_GetLocaleEncodingObject();
+        }
+    }
     if (encoding_str == NULL) {
         Py_CLEAR(buf);
         goto error;