From fc1917704069839a6f845cfb8063990ccbda089e Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Thu, 8 May 2025 21:31:17 +0100
Subject: [PATCH 1/9] Enable UTF-8 mode by default (PEP 686)

---
 Doc/c-api/init_config.rst                     |  4 +-
 Doc/library/os.rst                            | 37 +++++++++----------
 Doc/library/sys.rst                           |  3 ++
 Doc/using/windows.rst                         | 29 +++++++++------
 Doc/whatsnew/3.15.rst                         |  4 ++
 Include/cpython/initconfig.h                  | 13 +++----
 Lib/locale.py                                 |  3 +-
 Lib/subprocess.py                             |  3 +-
 Lib/test/test_cmd_line.py                     |  6 ++-
 Lib/test/test_embed.py                        | 10 +----
 Lib/test/test_utf8_mode.py                    |  6 +--
 ...-05-08-22-19-10.gh-issue-133711.e91wUy.rst |  2 +
 Programs/_testembed.c                         |  4 +-
 Python/initconfig.c                           |  2 +-
 Python/preconfig.c                            | 32 ++++------------
 15 files changed, 74 insertions(+), 84 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2025-05-08-22-19-10.gh-issue-133711.e91wUy.rst

diff --git a/Doc/c-api/init_config.rst b/Doc/c-api/init_config.rst
index e1931655618b1c..9273dcd028119c 100644
--- a/Doc/c-api/init_config.rst
+++ b/Doc/c-api/init_config.rst
@@ -975,9 +975,7 @@ PyPreConfig
       Set to ``0`` or ``1`` by the :option:`-X utf8 <-X>` command line option
       and the :envvar:`PYTHONUTF8` environment variable.
 
-      Also set to ``1`` if the ``LC_CTYPE`` locale is ``C`` or ``POSIX``.
-
-      Default: ``-1`` in Python config and ``0`` in isolated config.
+      Default: ``1``.
 
 
 .. _c-preinit:
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 1e54cfec609bd2..f07d075342ad7d 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -108,6 +108,12 @@ Python UTF-8 Mode
 .. versionadded:: 3.7
    See :pep:`540` for more details.
 
+.. versionchanged:: next
+
+   Python UTF-8 mode is now enabled by default (:pep:`686`).
+   It may be disabled with by setting :envvar:`PYTHONUTF8=0 <PYTHONUTF8>` as
+   an environment variable or by using the :option:`-X utf8=0 <-X>` flag.
+
 The Python UTF-8 Mode ignores the :term:`locale encoding` and forces the usage
 of the UTF-8 encoding:
 
@@ -139,31 +145,22 @@ level APIs also exhibit different default behaviours:
   default so that attempting to open a binary file in text mode is likely
   to raise an exception rather than producing nonsense data.
 
-The :ref:`Python UTF-8 Mode <utf8-mode>` is enabled if the LC_CTYPE locale is
-``C`` or ``POSIX`` at Python startup (see the :c:func:`PyConfig_Read`
-function).
-
-It can be enabled or disabled using the :option:`-X utf8 <-X>` command line
-option and the :envvar:`PYTHONUTF8` environment variable.
-
-If the :envvar:`PYTHONUTF8` environment variable is not set at all, then the
-interpreter defaults to using the current locale settings, *unless* the current
-locale is identified as a legacy ASCII-based locale (as described for
-:envvar:`PYTHONCOERCECLOCALE`), and locale coercion is either disabled or
-fails. In such legacy locales, the interpreter will default to enabling UTF-8
-mode unless explicitly instructed not to do so.
-
-The Python UTF-8 Mode can only be enabled at the Python startup. Its value
+The :ref:`Python UTF-8 Mode <utf8-mode>` is enabled by default.
+It can be disabled using the :option:`-X utf8 <-X>` command line
+option or the :envvar:`PYTHONUTF8` environment variable.
+The Python UTF-8 Mode can only be disabled at Python startup. Its value
 can be read from :data:`sys.flags.utf8_mode <sys.flags>`.
 
+If the UTF-8 mode is disabled, the interpreter defaults to using
+the current locale settings, *unless* the current locale is identified
+as a legacy ASCII-based locale (as described for :envvar:`PYTHONCOERCECLOCALE`),
+and locale coercion is either disabled or fails.
+In such legacy locales, the interpreter will default to enabling UTF-8 mode
+unless explicitly instructed not to do so.
+
 See also the :ref:`UTF-8 mode on Windows <win-utf8-mode>`
 and the :term:`filesystem encoding and error handler`.
 
-.. seealso::
-
-   :pep:`686`
-      Python 3.15 will make :ref:`utf8-mode` default.
-
 
 .. _os-procinfo:
 
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 55e442b20ff877..bf51ecfd5a3b7c 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -642,6 +642,9 @@ always available. Unless explicitly noted otherwise, all variables are read-only
    .. versionchanged:: 3.14
       Added the ``context_aware_warnings`` attribute.
 
+   .. versionchanged:: next
+      UTF-8 mode (:option:`-X utf8 <-X>`) is now enabled by default.
+
 
 .. data:: float_info
 
diff --git a/Doc/using/windows.rst b/Doc/using/windows.rst
index c084392ca66d55..471704564cf9f7 100644
--- a/Doc/using/windows.rst
+++ b/Doc/using/windows.rst
@@ -952,6 +952,9 @@ UTF-8 mode
 ==========
 
 .. versionadded:: 3.7
+.. versionchanged:: next
+
+   Python UTF-8 mode is now enabled by default (:pep:`686`).
 
 Windows still uses legacy encodings for the system encoding (the ANSI Code
 Page).  Python uses it for the default encoding of text files (e.g.
@@ -960,20 +963,22 @@ Page).  Python uses it for the default encoding of text files (e.g.
 This may cause issues because UTF-8 is widely used on the internet
 and most Unix systems, including WSL (Windows Subsystem for Linux).
 
-You can use the :ref:`Python UTF-8 Mode <utf8-mode>` to change the default text
-encoding to UTF-8. You can enable the :ref:`Python UTF-8 Mode <utf8-mode>` via
-the ``-X utf8`` command line option, or the ``PYTHONUTF8=1`` environment
-variable.  See :envvar:`PYTHONUTF8` for enabling UTF-8 mode, and
-:ref:`setting-envvars` for how to modify environment variables.
-
-When the :ref:`Python UTF-8 Mode <utf8-mode>` is enabled, you can still use the
+The :ref:`Python UTF-8 Mode <utf8-mode>`, enabled by default, can help by
+changing the default text encoding to UTF-8.
+When the :ref:`UTF-8 mode <utf8-mode>` is enabled, you can still use the
 system encoding (the ANSI Code Page) via the "mbcs" codec.
 
-Note that adding ``PYTHONUTF8=1`` to the default environment variables
-will affect all Python 3.7+ applications on your system.
-If you have any Python 3.7+ applications which rely on the legacy
-system encoding, it is recommended to set the environment variable
-temporarily or use the ``-X utf8`` command line option.
+You can disable the :ref:`Python UTF-8 Mode <utf8-mode>` via
+the ``-X utf8=0`` command line option, or the ``PYTHONUTF8=0`` environment
+variable.  See :envvar:`PYTHONUTF8` for disabling UTF-8 mode, and
+:ref:`setting-envvars` for how to modify environment variables.
+
+.. hint::
+   Adding ``PYTHONUTF8={0,1}`` to the default environment variables
+   will affect all Python 3.7+ applications on your system.
+   If you have any Python 3.7+ applications which rely on the legacy
+   system encoding, it is recommended to set the environment variable
+   temporarily or use the ``-X utf8`` command line option.
 
 .. note::
    Even when UTF-8 mode is disabled, Python uses UTF-8 by default
diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index 7131eeb697eb69..7f0a928d894a63 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -75,6 +75,10 @@ New features
 Other language changes
 ======================
 
+* Python UTF-8 mode is now enabled by default.
+  It may be disabled with by setting :envvar:`PYTHONUTF8=0 <PYTHONUTF8>` as
+  an environment variable or by using the :option:`-X utf8=0 <-X>` flag.
+  See :pep:`686` for further details.
 
 
 New modules
diff --git a/Include/cpython/initconfig.h b/Include/cpython/initconfig.h
index 7ce4acfeb7177d..7ab0f8de004d18 100644
--- a/Include/cpython/initconfig.h
+++ b/Include/cpython/initconfig.h
@@ -102,15 +102,14 @@ typedef struct PyPreConfig {
 
     /* Enable UTF-8 mode? (PEP 540)
 
-       Disabled by default (equals to 0).
+      If equal to 1, use the UTF-8 encoding and use "surrogateescape" for the
+      stdin & stdout error handlers.
 
-       Set to 1 by "-X utf8" and "-X utf8=1" command line options.
-       Set to 1 by PYTHONUTF8=1 environment variable.
+       Enabled by default (equal to 1; PEP 686), or if Py_UTF8Mode=1,
+       or if "-X utf8=1" or PYTHONUTF8=1.
 
-       Set to 0 by "-X utf8=0" and PYTHONUTF8=0.
-
-       If equals to -1, it is set to 1 if the LC_CTYPE locale is "C" or
-       "POSIX", otherwise it is set to 0. Inherit Py_UTF8Mode value value. */
+       Set to 0 by "-X utf8=0" or PYTHONUTF8=0.
+    */
     int utf8_mode;
 
     /* If non-zero, enable the Python Development Mode.
diff --git a/Lib/locale.py b/Lib/locale.py
index 2feb10e59c96a3..4e2221c36e19e5 100644
--- a/Lib/locale.py
+++ b/Lib/locale.py
@@ -651,7 +651,8 @@ def getpreferredencoding(do_setlocale=True):
         if sys.flags.warn_default_encoding:
             import warnings
             warnings.warn(
-                "UTF-8 Mode affects locale.getpreferredencoding(). Consider locale.getencoding() instead.",
+                "UTF-8 Mode affects locale.getpreferredencoding(). "
+                "Consider locale.getencoding() instead.",
                 EncodingWarning, 2)
         if sys.flags.utf8_mode:
             return 'utf-8'
diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index 54c2eb515b60da..79251bd5310223 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -380,8 +380,7 @@ def _text_encoding():
 
     if sys.flags.utf8_mode:
         return "utf-8"
-    else:
-        return locale.getencoding()
+    return locale.getencoding()
 
 
 def call(*popenargs, timeout=None, **kwargs):
diff --git a/Lib/test/test_cmd_line.py b/Lib/test/test_cmd_line.py
index 1b40e0d05fe3bc..92f4228109f7e6 100644
--- a/Lib/test/test_cmd_line.py
+++ b/Lib/test/test_cmd_line.py
@@ -299,6 +299,10 @@ def run_utf8_mode(arg):
             cmd = [sys.executable, '-X', 'utf8', '-c', code, arg]
             return subprocess.run(cmd, stdout=subprocess.PIPE, text=True)
 
+        def run_no_utf8_mode(arg):
+            cmd = [sys.executable, '-X', 'utf8=0', '-c', code, arg]
+            return subprocess.run(cmd, stdout=subprocess.PIPE, text=True)
+
         valid_utf8 = 'e:\xe9, euro:\u20ac, non-bmp:\U0010ffff'.encode('utf-8')
         # invalid UTF-8 byte sequences with a valid UTF-8 sequence
         # in the middle.
@@ -311,7 +315,7 @@ def run_utf8_mode(arg):
         )
         test_args = [valid_utf8, invalid_utf8]
 
-        for run_cmd in (run_default, run_c_locale, run_utf8_mode):
+        for run_cmd in (run_default, run_c_locale, run_utf8_mode, run_no_utf8_mode):
             with self.subTest(run_cmd=run_cmd):
                 for arg in test_args:
                     proc = run_cmd(arg)
diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
index 95b2d80464c349..c53fdc717f737f 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -543,7 +543,7 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
         'configure_locale': True,
         'coerce_c_locale': False,
         'coerce_c_locale_warn': False,
-        'utf8_mode': False,
+        'utf8_mode': True,
     }
     if MS_WINDOWS:
         PRE_CONFIG_COMPAT.update({
@@ -560,7 +560,7 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
         configure_locale=False,
         isolated=True,
         use_environment=False,
-        utf8_mode=False,
+        utf8_mode=True,
         dev_mode=False,
         coerce_c_locale=False,
     )
@@ -805,12 +805,6 @@ def get_expected_config(self, expected_preconfig, expected,
                         'stdio_encoding', 'stdio_errors'):
                 expected[key] = self.IGNORE_CONFIG
 
-        if not expected_preconfig['configure_locale']:
-            # UTF-8 Mode depends on the locale. There is no easy way
-            # to guess if UTF-8 Mode will be enabled or not if the locale
-            # is not configured.
-            expected_preconfig['utf8_mode'] = self.IGNORE_CONFIG
-
         if expected_preconfig['utf8_mode'] == 1:
             if expected['filesystem_encoding'] is self.GET_DEFAULT_CONFIG:
                 expected['filesystem_encoding'] = 'utf-8'
diff --git a/Lib/test/test_utf8_mode.py b/Lib/test/test_utf8_mode.py
index f66881044e16df..b8e49440c9f7da 100644
--- a/Lib/test/test_utf8_mode.py
+++ b/Lib/test/test_utf8_mode.py
@@ -89,8 +89,8 @@ def test_env_var(self):
         # the UTF-8 mode
         if not self.posix_locale():
             # PYTHONUTF8 should be ignored if -E is used
-            out = self.get_output('-E', '-c', code, PYTHONUTF8='1')
-            self.assertEqual(out, '0')
+            out = self.get_output('-E', '-c', code, PYTHONUTF8='0')
+            self.assertEqual(out, '1')
 
         # invalid mode
         out = self.get_output('-c', code, PYTHONUTF8='xxx', failure=True)
@@ -116,7 +116,7 @@ def test_filesystemencoding(self):
             # PYTHONLEGACYWINDOWSFSENCODING disables the UTF-8 mode
             # and has the priority over -X utf8 and PYTHONUTF8
             out = self.get_output('-X', 'utf8', '-c', code,
-                                  PYTHONUTF8='strict',
+                                  PYTHONUTF8='xxx',
                                   PYTHONLEGACYWINDOWSFSENCODING='1')
             self.assertEqual(out, 'mbcs/replace')
 
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-05-08-22-19-10.gh-issue-133711.e91wUy.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-08-22-19-10.gh-issue-133711.e91wUy.rst
new file mode 100644
index 00000000000000..c8d3d62763dc12
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-08-22-19-10.gh-issue-133711.e91wUy.rst
@@ -0,0 +1,2 @@
+Implement :pep:`686`: Enable :ref:`Python UTF-8 Mode <utf8-mode>` by
+default. Patch by Adam Turner.
diff --git a/Programs/_testembed.c b/Programs/_testembed.c
index 8a7412c7019e60..cfd35c7da5e6c7 100644
--- a/Programs/_testembed.c
+++ b/Programs/_testembed.c
@@ -1882,9 +1882,9 @@ static int test_initconfig_get_api(void)
     assert(initconfig_getint(config, "dev_mode") == 1);
 
     // test PyInitConfig_GetInt() on a PyPreConfig option
-    assert(initconfig_getint(config, "utf8_mode") == 0);
-    assert(PyInitConfig_SetInt(config, "utf8_mode", 1) == 0);
     assert(initconfig_getint(config, "utf8_mode") == 1);
+    assert(PyInitConfig_SetInt(config, "utf8_mode", 0) == 0);
+    assert(initconfig_getint(config, "utf8_mode") == 0);
 
     // test PyInitConfig_GetStr()
     char *str;
diff --git a/Python/initconfig.c b/Python/initconfig.c
index e827091172162e..8f974aeab4beaa 100644
--- a/Python/initconfig.c
+++ b/Python/initconfig.c
@@ -459,7 +459,7 @@ static const char usage_envvars[] =
 
 /* --- Global configuration variables ----------------------------- */
 
-/* UTF-8 mode (PEP 540): if equals to 1, use the UTF-8 encoding, and change
+/* UTF-8 mode (PEP 540): if equal to 1, use the UTF-8 encoding, and change
    stdin and stdout error handler to "surrogateescape". */
 int Py_UTF8Mode = 0;
 int Py_DebugFlag = 0; /* Needed by parser.c */
diff --git a/Python/preconfig.c b/Python/preconfig.c
index 5b26c75de8b3a0..7b3270d5036951 100644
--- a/Python/preconfig.c
+++ b/Python/preconfig.c
@@ -291,12 +291,12 @@ _PyPreConfig_InitCompatConfig(PyPreConfig *config)
     config->use_environment = -1;
     config->configure_locale = 1;
 
-    /* bpo-36443: C locale coercion (PEP 538) and UTF-8 Mode (PEP 540)
-       are disabled by default using the Compat configuration.
+    /* gh-80624: C locale coercion (PEP 538) is disabled by default using
+       the Compat configuration.
 
-       Py_UTF8Mode=1 enables the UTF-8 mode. PYTHONUTF8 environment variable
+       Py_UTF8Mode=0 disables the UTF-8 mode. PYTHONUTF8 environment variable
        is ignored (even if use_environment=1). */
-    config->utf8_mode = 0;
+    config->utf8_mode = 1;
     config->coerce_c_locale = 0;
     config->coerce_c_locale_warn = 0;
 
@@ -317,8 +317,8 @@ PyPreConfig_InitPythonConfig(PyPreConfig *config)
     config->isolated = 0;
     config->parse_argv = 1;
     config->use_environment = 1;
-    /* Set to -1 to enable C locale coercion (PEP 538) and UTF-8 Mode (PEP 540)
-       depending on the LC_CTYPE locale, PYTHONUTF8 and PYTHONCOERCECLOCALE
+    /* Set to -1 to enable C locale coercion (PEP 538) depending on
+       the LC_CTYPE locale, PYTHONUTF8 and PYTHONCOERCECLOCALE
        environment variables. */
     config->coerce_c_locale = -1;
     config->coerce_c_locale_warn = -1;
@@ -338,7 +338,7 @@ PyPreConfig_InitIsolatedConfig(PyPreConfig *config)
     config->configure_locale = 0;
     config->isolated = 1;
     config->use_environment = 0;
-    config->utf8_mode = 0;
+    config->utf8_mode = 1;
     config->dev_mode = 0;
 #ifdef MS_WINDOWS
     config->legacy_windows_fs_encoding = 0;
@@ -649,23 +649,7 @@ preconfig_init_utf8_mode(PyPreConfig *config, const _PyPreCmdline *cmdline)
         return _PyStatus_OK();
     }
 
-
-#ifndef MS_WINDOWS
-    if (config->utf8_mode < 0) {
-        /* The C locale and the POSIX locale enable the UTF-8 Mode (PEP 540) */
-        const char *ctype_loc = setlocale(LC_CTYPE, NULL);
-        if (ctype_loc != NULL
-           && (strcmp(ctype_loc, "C") == 0
-               || strcmp(ctype_loc, "POSIX") == 0))
-        {
-            config->utf8_mode = 1;
-        }
-    }
-#endif
-
-    if (config->utf8_mode < 0) {
-        config->utf8_mode = 0;
-    }
+    config->utf8_mode = 1;
     return _PyStatus_OK();
 }
 

From c706cfaf1491faea0224f1a012ebe6433c67bb5a Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sat, 10 May 2025 01:47:45 +0100
Subject: [PATCH 2/9] Victor's documentation comments

---
 Doc/library/os.rst  | 4 ++--
 Doc/library/sys.rst | 3 ---
 2 files changed, 2 insertions(+), 5 deletions(-)

diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index f07d075342ad7d..ac0bff30ed80e7 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -146,8 +146,8 @@ level APIs also exhibit different default behaviours:
   to raise an exception rather than producing nonsense data.
 
 The :ref:`Python UTF-8 Mode <utf8-mode>` is enabled by default.
-It can be disabled using the :option:`-X utf8 <-X>` command line
-option or the :envvar:`PYTHONUTF8` environment variable.
+It can be disabled using the :option:`-X utf8=0 <-X>` command line
+option or the :envvar:`PYTHONUTF8=0 <PYTHONUTF8>` environment variable.
 The Python UTF-8 Mode can only be disabled at Python startup. Its value
 can be read from :data:`sys.flags.utf8_mode <sys.flags>`.
 
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index bf51ecfd5a3b7c..55e442b20ff877 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -642,9 +642,6 @@ always available. Unless explicitly noted otherwise, all variables are read-only
    .. versionchanged:: 3.14
       Added the ``context_aware_warnings`` attribute.
 
-   .. versionchanged:: next
-      UTF-8 mode (:option:`-X utf8 <-X>`) is now enabled by default.
-
 
 .. data:: float_info
 

From f095ab00b65ab7f30127bbd00ee116efe8643d31 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sat, 10 May 2025 02:29:10 +0100
Subject: [PATCH 3/9] Fix test_cmd_line

---
 Lib/test/test_cmd_line.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Lib/test/test_cmd_line.py b/Lib/test/test_cmd_line.py
index 92f4228109f7e6..c83bf23692007c 100644
--- a/Lib/test/test_cmd_line.py
+++ b/Lib/test/test_cmd_line.py
@@ -976,9 +976,10 @@ def test_python_legacy_windows_fs_encoding(self):
 
     @unittest.skipUnless(support.MS_WINDOWS, 'Test only applicable on Windows')
     def test_python_legacy_windows_stdio(self):
-        code = "import sys; print(sys.stdin.encoding, sys.stdout.encoding)"
+        code = 'import sys; print(sys.stdin.encoding, sys.stdout.encoding)'
         expected = 'cp'
-        rc, out, err = assert_python_ok('-c', code, PYTHONLEGACYWINDOWSSTDIO='1')
+        rc, out, err = assert_python_ok('-Xutf8=0', '-c', code,
+                                        PYTHONLEGACYWINDOWSSTDIO='1')
         self.assertIn(expected.encode(), out)
 
     @unittest.skipIf("-fsanitize" in sysconfig.get_config_vars().get('PY_CFLAGS', ()),

From ff066b1b12f4a4887843ed6d6d372d1d93594d1e Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sat, 10 May 2025 03:08:35 +0100
Subject: [PATCH 4/9] Improve What's New entry

---
 Doc/whatsnew/3.15.rst | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index 7f0a928d894a63..2d3a15405de337 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -75,11 +75,24 @@ New features
 Other language changes
 ======================
 
-* Python UTF-8 mode is now enabled by default.
-  It may be disabled with by setting :envvar:`PYTHONUTF8=0 <PYTHONUTF8>` as
-  an environment variable or by using the :option:`-X utf8=0 <-X>` flag.
-  See :pep:`686` for further details.
+* Python now uses UTF-8 as the default encoding, independent of the system's
+  environment. This means that I/O operations without an explicit encoding,
+  e.g. ``open('flying-circus.txt')``, will use UTF-8.
 
+  This only applies when no ``encoding`` argument is given. For best
+  compatibility between versions of Python, ensure that an explicit ``encoding``
+  argument is always provided. The :ref:`opt-in encoding warning <io-encoding-warning>`
+  can be used to identify code that may be affected by this change.
+  The special special ``encoding='locale'`` argument uses the current locale
+  encoding, and has been supported since Python 3.10.
+
+  To retain the previous behaviour, Python's UTF-8 mode may be disabled with
+  the :envvar:`PYTHONUTF8=0 <PYTHONUTF8>` environment variable or the
+  :option:`-X utf8=0 <-X>` command line option.
+
+  .. seealso:: :pep:`686` for further details.
+
+  (Contributed by Adam Turner in :gh:`133711`; PEP 686 written by Inada Naoki.)
 
 New modules
 ===========

From abfb86419453cdf837de5d5d18a17797cb3ed130 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sat, 10 May 2025 03:20:01 +0100
Subject: [PATCH 5/9] Add references to UTF-8 and Unicode

---
 Doc/whatsnew/3.15.rst | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index 2d3a15405de337..2484f8eaa8431c 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -75,9 +75,12 @@ New features
 Other language changes
 ======================
 
-* Python now uses UTF-8 as the default encoding, independent of the system's
+* Python now uses UTF-8_ as the default encoding, independent of the system's
   environment. This means that I/O operations without an explicit encoding,
   e.g. ``open('flying-circus.txt')``, will use UTF-8.
+  UTF-8 is a widely-supported Unicode_ character encoding that has become a
+  *de facto* standard for representing text, including nearly every webpage
+  on the internet, many common file formats, programming languages, and more.
 
   This only applies when no ``encoding`` argument is given. For best
   compatibility between versions of Python, ensure that an explicit ``encoding``
@@ -92,6 +95,9 @@ Other language changes
 
   .. seealso:: :pep:`686` for further details.
 
+  .. _UTF-8: https://en.wikipedia.org/wiki/UTF-8
+  .. _Unicode: https://home.unicode.org/
+
   (Contributed by Adam Turner in :gh:`133711`; PEP 686 written by Inada Naoki.)
 
 New modules

From 3226e1d9dd16b9ec2b41bd3a261de3d46c29af45 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Sat, 10 May 2025 23:50:53 +0100
Subject: [PATCH 6/9] Check stream buffer type in
 test_python_legacy_windows_stdio

---
 Lib/test/test_cmd_line.py | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/Lib/test/test_cmd_line.py b/Lib/test/test_cmd_line.py
index c83bf23692007c..64db09efcffed3 100644
--- a/Lib/test/test_cmd_line.py
+++ b/Lib/test/test_cmd_line.py
@@ -976,11 +976,19 @@ def test_python_legacy_windows_fs_encoding(self):
 
     @unittest.skipUnless(support.MS_WINDOWS, 'Test only applicable on Windows')
     def test_python_legacy_windows_stdio(self):
-        code = 'import sys; print(sys.stdin.encoding, sys.stdout.encoding)'
-        expected = 'cp'
-        rc, out, err = assert_python_ok('-Xutf8=0', '-c', code,
-                                        PYTHONLEGACYWINDOWSSTDIO='1')
-        self.assertIn(expected.encode(), out)
+        def get_stderr_class(legacy_windows_stdio):
+            code = 'import sys; print(type(sys.stderr.buffer.raw))'
+            env = {'PYTHONLEGACYWINDOWSSTDIO': str(int(legacy_windows_stdio))}
+            # use stderr=None as legacy_windows_stdio doesn't affect pipes
+            p = spawn_python('-c', code, env=env, stderr=None)
+            out = kill_python(p).strip().decode('ascii', 'ignore')
+            return out.removeprefix("<class '").removesuffix("'>")
+
+        out = get_stderr_class(legacy_windows_stdio=True)
+        self.assertEqual('_io.FileIO', out)
+
+        out = get_stderr_class(legacy_windows_stdio=False)
+        self.assertEqual('_io._WindowsConsoleIO', out)
 
     @unittest.skipIf("-fsanitize" in sysconfig.get_config_vars().get('PY_CFLAGS', ()),
                      "PYTHONMALLOCSTATS doesn't work with ASAN")

From 3da6bf2976cc8dfb40c7da1077d397adf0cf2c10 Mon Sep 17 00:00:00 2001
From: Adam Turner <9087854+aa-turner@users.noreply.github.com>
Date: Mon, 26 May 2025 10:36:09 +0100
Subject: [PATCH 7/9] line length

---
 Lib/test/test_cmd_line.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Lib/test/test_cmd_line.py b/Lib/test/test_cmd_line.py
index 03cbb7530690ca..f30a1874ab96d4 100644
--- a/Lib/test/test_cmd_line.py
+++ b/Lib/test/test_cmd_line.py
@@ -316,7 +316,8 @@ def run_no_utf8_mode(arg):
         )
         test_args = [valid_utf8, invalid_utf8]
 
-        for run_cmd in (run_default, run_c_locale, run_utf8_mode, run_no_utf8_mode):
+        for run_cmd in (run_default, run_c_locale, run_utf8_mode,
+                        run_no_utf8_mode):
             with self.subTest(run_cmd=run_cmd):
                 for arg in test_args:
                     proc = run_cmd(arg)

From 6ecea6902457cfbab50058dcc0c34ef371ea5c3f Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Sun, 13 Jul 2025 19:01:03 +0200
Subject: [PATCH 8/9] Update Doc/library/os.rst

---
 Doc/library/os.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index ac0bff30ed80e7..45ec6c7a51b7b0 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -112,7 +112,7 @@ Python UTF-8 Mode
 
    Python UTF-8 mode is now enabled by default (:pep:`686`).
    It may be disabled with by setting :envvar:`PYTHONUTF8=0 <PYTHONUTF8>` as
-   an environment variable or by using the :option:`-X utf8=0 <-X>` flag.
+   an environment variable or by using the :option:`-X utf8=0 <-X>` command line option.
 
 The Python UTF-8 Mode ignores the :term:`locale encoding` and forces the usage
 of the UTF-8 encoding:

From e87ce6c0a37d191c73b71dac57cefe467b75d560 Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Sun, 13 Jul 2025 19:01:23 +0200
Subject: [PATCH 9/9] Update Include/cpython/initconfig.h

---
 Include/cpython/initconfig.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Include/cpython/initconfig.h b/Include/cpython/initconfig.h
index 7ab0f8de004d18..1c979d91a40850 100644
--- a/Include/cpython/initconfig.h
+++ b/Include/cpython/initconfig.h
@@ -105,8 +105,8 @@ typedef struct PyPreConfig {
       If equal to 1, use the UTF-8 encoding and use "surrogateescape" for the
       stdin & stdout error handlers.
 
-       Enabled by default (equal to 1; PEP 686), or if Py_UTF8Mode=1,
-       or if "-X utf8=1" or PYTHONUTF8=1.
+      Enabled by default (equal to 1; PEP 686), or if Py_UTF8Mode=1,
+      or if "-X utf8=1" or PYTHONUTF8=1.
 
        Set to 0 by "-X utf8=0" or PYTHONUTF8=0.
     */