python · serhiy-storchaka · Aug 9, 2026
diff --git a/Lib/test/test_cmd_line.py b/Lib/test/test_cmd_line.py
@@ -1067,6 +1067,41 @@ 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 the standard streams
+        # is the encoding of the console.
+        import ctypes
+        kernel32 = ctypes.WinDLL('kernel32', use_last_error=True)
+        try:
+            fin = open('CONIN$')
+        except OSError:
+            self.skipTest('no console')
+        # We cannot use PIPE, because the standard streams should be
+        # connected to the console.  So we use the exit code.
+        code = ("import sys; sys.exit(sys.stdin.encoding != 'cp850' or "
+                "sys.stdout.encoding != 'cp850')")
+        env = os.environ.copy()
+        env['PYTHONLEGACYWINDOWSSTDIO'] = '1'
+        env['PYTHONUTF8'] = '0'
+        env.pop('PYTHONIOENCODING', None)
+        old_cp = kernel32.GetConsoleCP()
+        old_output_cp = kernel32.GetConsoleOutputCP()
+        with fin, open('CONOUT$', 'w') as fout:
+            try:
+                if not kernel32.SetConsoleCP(850):
+                    self.skipTest('cannot set the console input code page')
+                if not kernel32.SetConsoleOutputCP(850):
+                    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/2026-08-09-07-00-00.gh-issue-86427.consolecp.rst b/Misc/NEWS.d/next/Windows/2026-08-09-07-00-00.gh-issue-86427.consolecp.rst
@@ -0,0 +1,3 @@
+Fix the encoding of the standard streams in the legacy Windows stdio mode
+(:envvar:`PYTHONLEGACYWINDOWSSTDIO`).  It is now the code page of the
+console, as in Python 3.7, not the ANSI code page.
@@ -2710,8 +2710,30 @@ config_init_stdio_encoding(PyConfig *config,
 
     /* Choose the default error handler based on the current locale. */
     if (config->stdio_encoding == NULL) {
-        status = config_get_locale_encoding(config, preconfig,
-                                            &config->stdio_encoding);
+#ifdef MS_WINDOWS
+        /* gh-86427: use the console code page.  Only one encoding can be
+           specified, so the output code page is used: it affects two
+           streams of three. */
+        UINT cp = config->legacy_windows_stdio ? GetConsoleOutputCP() : 0;
+        if (cp != 0) {
+            if (cp == CP_UTF8) {
+                status = PyConfig_SetString(config, &config->stdio_encoding,
+                                            L"utf-8");
+            }
+            else {
+                wchar_t encoding[20];
+                swprintf(encoding, Py_ARRAY_LENGTH(encoding), L"cp%u",
+                         (unsigned int)cp);
+                status = PyConfig_SetString(config, &config->stdio_encoding,
+                                            encoding);
+            }
+        }
+        else
+#endif
+        {
+            status = config_get_locale_encoding(config, preconfig,
+                                                &config->stdio_encoding);
+        }
         if (_PyStatus_EXCEPTION(status)) {
             return status;
         }