python · serhiy-storchaka · Aug 9, 2026
diff --git a/Lib/test/test_cmd_line.py b/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):

diff --git a/Misc/NEWS.d/next/Windows/2026-08-09-05-30-00.gh-issue-86427.legacystdio.rst b/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.
diff --git a/Objects/unicodeobject.c b/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");

@@ -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)) {

@@ -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;