GH-133711: Enable UTF-8 mode by default (PEP 686)

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:

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>` command line option.
+
 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=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>`.
 
+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:
 

Doc/using/windows.rst

@@ -1006,6 +1006,9 @@ 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.
@@ -1014,20 +1017,22 @@ 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

Doc/whatsnew/3.15.rst

@@ -172,11 +172,35 @@ production systems where traditional profiling approaches would be too intrusive
 Other language changes
 ======================
 
+* 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``
+ 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.
+
+ .. _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.)
+
 * Several error messages incorrectly using the term "argument" have been corrected.
  (Contributed by Stan Ulbrych in :gh:`133382`.)
 
 
-
 New modules
 ===========
 

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.

Lib/locale.py

@@ -651,7 +651,8 @@ def getpreferredencoding(do_setlocale=True):
 ifsys.flags.warn_default_encoding:
 importwarnings
 warnings.warn(
-"UTF-8 Mode affects locale.getpreferredencoding(). Consider locale.getencoding() instead.",
+"UTF-8 Mode affects locale.getpreferredencoding(). "
+"Consider locale.getencoding() instead.",
 EncodingWarning, 2)
 ifsys.flags.utf8_mode:
 return'utf-8'

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):

Lib/test/test_cmd_line.py

@@ -300,6 +300,10 @@ 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.
@@ -312,7 +316,8 @@ 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)

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'

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')
 

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.

Programs/_testembed.c

@@ -1854,9 +1854,9 @@ 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;

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 */

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();
 }