gh-86427: Replace PyConfig.stdio_encoding with three separate members

PyConfig.stdin_encoding, PyConfig.stdout_encoding and
PyConfig.stderr_encoding allow the standard streams to have different
encodings.

In the legacy Windows stdio mode they are initialized with the encoding of
the device the corresponding stream is connected to, instead of the ANSI
code page.

Author: serhiy-storchaka

Doc/c-api/init_config.rst

@@ -477,8 +477,12 @@ Configuration Options
      - :c:member:`skip_source_first_line <PyConfig.skip_source_first_line>`
      - ``bool``
      - Read-only
-   * - ``"stdio_encoding"``
-     - :c:member:`stdio_encoding <PyConfig.stdio_encoding>`
+   * - ``"stderr_encoding"``
+     - :c:member:`stderr_encoding <PyConfig.stderr_encoding>`
+   * - ``"stdin_encoding"``
+     - :c:member:`stdin_encoding <PyConfig.stdin_encoding>`
+   * - ``"stdout_encoding"``
+     - :c:member:`stdout_encoding <PyConfig.stdout_encoding>`
      - ``str``
      - Read-only
    * - ``"stdio_errors"``
@@ -1863,12 +1867,14 @@ PyConfig
 
       Default: ``0``.
 
-   .. c:member:: wchar_t* stdio_encoding
+   .. c:member:: wchar_t* stdin_encoding
+   .. c:member:: wchar_t* stdout_encoding
+   .. c:member:: wchar_t* stderr_encoding
    .. c:member:: wchar_t* stdio_errors
 
-      Encoding and encoding errors of :data:`sys.stdin`, :data:`sys.stdout` and
-      :data:`sys.stderr` (but :data:`sys.stderr` always uses
-      ``"backslashreplace"`` error handler).
+      Encoding of :data:`sys.stdin`, :data:`sys.stdout` and :data:`sys.stderr`
+      respectively, and encoding errors of all of them (but :data:`sys.stderr`
+      always uses the ``"backslashreplace"`` error handler).
 
       Use the :envvar:`PYTHONIOENCODING` environment variable if it is
       non-empty.

Doc/library/sys.rst

@@ -2139,7 +2139,8 @@ always available. Unless explicitly noted otherwise, all variables are read-only
    follows:
 
    * The encoding and error handling are initialized from
-     :c:member:`PyConfig.stdio_encoding` and :c:member:`PyConfig.stdio_errors`.
+     :c:member:`PyConfig.stdin_encoding`, :c:member:`PyConfig.stdout_encoding`,
+     :c:member:`PyConfig.stderr_encoding` and :c:member:`PyConfig.stdio_errors`.
 
      On Windows, UTF-8 is used for the console device.  Non-character
      devices such as disk files and pipes use the system locale

Include/cpython/initconfig.h

@@ -171,7 +171,9 @@ typedef struct PyConfig {
     int user_site_directory;
     int configure_c_stdio;
     int buffered_stdio;
-    wchar_t *stdio_encoding;
+    wchar_t *stdin_encoding;
+    wchar_t *stdout_encoding;
+    wchar_t *stderr_encoding;
     wchar_t *stdio_errors;
 #ifdef MS_WINDOWS
     int legacy_windows_stdio;

Lib/test/test_capi/test_config.py

@@ -86,7 +86,9 @@ def test_config_get(self):
             ("show_ref_count", bool, None),
             ("site_import", bool, None),
             ("skip_source_first_line", bool, None),
-            ("stdio_encoding", str, None),
+            ("stderr_encoding", str, None),
+            ("stdin_encoding", str, None),
+            ("stdout_encoding", str, None),
             ("stdio_errors", str, None),
             ("stdlib_dir", str | None, "_stdlib_dir"),
             ("tracemalloc", int, None),

Lib/test/test_cmd_line.py

@@ -1069,8 +1069,9 @@ def test_python_legacy_windows_stdio(self):
 
     @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.
+        # 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:
@@ -1080,7 +1081,7 @@ def test_python_legacy_windows_stdio_encoding(self):
         # 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')")
+                "sys.stdout.encoding != 'cp437')")
         env = os.environ.copy()
         env['PYTHONLEGACYWINDOWSSTDIO'] = '1'
         env['PYTHONUTF8'] = '0'
@@ -1091,7 +1092,7 @@ def test_python_legacy_windows_stdio_encoding(self):
             try:
                 if not kernel32.SetConsoleCP(850):
                     self.skipTest('cannot set the console input code page')
-                if not kernel32.SetConsoleOutputCP(850):
+                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,

Lib/test/test_embed.py

@@ -767,7 +767,9 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
         'configure_c_stdio': False,
         'buffered_stdio': True,
 
-        'stdio_encoding': GET_DEFAULT_CONFIG,
+        'stdin_encoding': GET_DEFAULT_CONFIG,
+        'stdout_encoding': GET_DEFAULT_CONFIG,
+        'stderr_encoding': GET_DEFAULT_CONFIG,
         'stdio_errors': GET_DEFAULT_CONFIG,
 
         'skip_source_first_line': False,
@@ -934,16 +936,19 @@ def get_expected_config(self, expected_preconfig, expected,
             # there is no easy way to get the locale encoding before
             # setlocale(LC_CTYPE, "") is called: don't test encodings
             for key in ('filesystem_encoding', 'filesystem_errors',
-                        'stdio_encoding', 'stdio_errors'):
+                        'stdin_encoding', 'stdout_encoding',
+                        'stderr_encoding', 'stdio_errors'):
                 expected[key] = self.IGNORE_CONFIG
 
         if expected_preconfig['utf8_mode'] == 1:
             if expected['filesystem_encoding'] is self.GET_DEFAULT_CONFIG:
                 expected['filesystem_encoding'] = 'utf-8'
             if expected['filesystem_errors'] is self.GET_DEFAULT_CONFIG:
                 expected['filesystem_errors'] = self.UTF8_MODE_ERRORS
-            if expected['stdio_encoding'] is self.GET_DEFAULT_CONFIG:
-                expected['stdio_encoding'] = 'utf-8'
+            for key in ('stdin_encoding', 'stdout_encoding',
+                        'stderr_encoding'):
+                if expected[key] is self.GET_DEFAULT_CONFIG:
+                    expected[key] = 'utf-8'
             if expected['stdio_errors'] is self.GET_DEFAULT_CONFIG:
                 expected['stdio_errors'] = 'surrogateescape'
 
@@ -1133,7 +1138,9 @@ def test_init_from_config(self):
             'malloc_stats': True,
             'pymalloc_hugepages': True,
 
-            'stdio_encoding': 'iso8859-1',
+            'stdin_encoding': 'iso8859-1',
+            'stdout_encoding': 'iso8859-1',
+            'stderr_encoding': 'iso8859-1',
             'stdio_errors': 'replace',
 
             'pycache_prefix': 'conf_pycache_prefix',
@@ -1205,7 +1212,9 @@ def test_init_compat_env(self):
             'write_bytecode': False,
             'verbose': 1,
             'buffered_stdio': False,
-            'stdio_encoding': 'iso8859-1',
+            'stdin_encoding': 'iso8859-1',
+            'stdout_encoding': 'iso8859-1',
+            'stderr_encoding': 'iso8859-1',
             'stdio_errors': 'replace',
             'user_site_directory': False,
             'faulthandler': True,
@@ -1242,7 +1251,9 @@ def test_init_python_env(self):
             'write_bytecode': False,
             'verbose': 1,
             'buffered_stdio': False,
-            'stdio_encoding': 'iso8859-1',
+            'stdin_encoding': 'iso8859-1',
+            'stdout_encoding': 'iso8859-1',
+            'stderr_encoding': 'iso8859-1',
             'stdio_errors': 'replace',
             'user_site_directory': False,
             'faulthandler': True,

Misc/NEWS.d/next/C_API/2026-08-09-06-30-00.gh-issue-86427.threeenc.rst

@@ -0,0 +1,5 @@
+Replace :c:member:`!PyConfig.stdio_encoding` with three separate members:
+:c:member:`PyConfig.stdin_encoding`, :c:member:`PyConfig.stdout_encoding` and
+:c:member:`PyConfig.stderr_encoding`.  In the legacy Windows stdio mode they
+are initialized with the encoding of the device the corresponding stream is
+connected to.

Misc/NEWS.d/next/Windows/2026-08-09-06-40-00.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.

Misc/NEWS.d/next/Windows/2026-08-09-07-00-00.gh-issue-86427.consolecp.rst

@@ -15209,9 +15209,14 @@ init_stdio_encoding(PyInterpreterState *interp)
 {
     /* Update the stdio encoding to the normalized Python codec name. */
     PyConfig *config = (PyConfig*)_PyInterpreterState_GetConfig(interp);
-    if (config_get_codec_name(&config->stdio_encoding) < 0) {
-        return _PyStatus_ERR("failed to get the Python codec name "
-                             "of the stdio encoding");
+    wchar_t **encodings[] = {&config->stdin_encoding,
+                             &config->stdout_encoding,
+                             &config->stderr_encoding};
+    for (size_t i = 0; i < Py_ARRAY_LENGTH(encodings); i++) {
+        if (config_get_codec_name(encodings[i]) < 0) {
+            return _PyStatus_ERR("failed to get the Python codec name "
+                                 "of the stdio encoding");
+        }
     }
     return _PyStatus_OK();
 }