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GH-133711: Enable UTF-8 mode by default (PEP 686) (#133712)
Co-authored-by: Victor Stinner <vstinner@python.org>
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‎Doc/c-api/init_config.rst

Lines changed: 1 addition & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -975,9 +975,7 @@ PyPreConfig
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Set to ``0`` or ``1`` by the :option:`-X utf8 <-X>` command line option
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and the :envvar:`PYTHONUTF8` environment variable.
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978-
Also set to ``1`` if the ``LC_CTYPE`` locale is ``C`` or ``POSIX``.
979-
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Default: ``-1`` in Python config and ``0`` in isolated config.
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Default: ``1``.
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.. _c-preinit:

‎Doc/library/os.rst

Lines changed: 17 additions & 20 deletions
Original file line numberDiff line numberDiff line change
@@ -108,6 +108,12 @@ Python UTF-8 Mode
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.. versionadded:: 3.7
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See :pep:`540` for more details.
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.. versionchanged:: next
112+
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Python UTF-8 mode is now enabled by default (:pep:`686`).
114+
It may be disabled with by setting :envvar:`PYTHONUTF8=0 <PYTHONUTF8>` as
115+
an environment variable or by using the :option:`-X utf8=0 <-X>` command line option.
116+
111117
The Python UTF-8 Mode ignores the :term:`locale encoding` and forces the usage
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of the UTF-8 encoding:
113119

@@ -139,31 +145,22 @@ level APIs also exhibit different default behaviours:
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default so that attempting to open a binary file in text mode is likely
140146
to raise an exception rather than producing nonsense data.
141147

142-
The :ref:`Python UTF-8 Mode <utf8-mode>` is enabled if the LC_CTYPE locale is
143-
``C`` or ``POSIX`` at Python startup (see the :c:func:`PyConfig_Read`
144-
function).
145-
146-
It can be enabled or disabled using the :option:`-X utf8 <-X>` command line
147-
option and the :envvar:`PYTHONUTF8` environment variable.
148-
149-
If the :envvar:`PYTHONUTF8` environment variable is not set at all, then the
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interpreter defaults to using the current locale settings, *unless* the current
151-
locale is identified as a legacy ASCII-based locale (as described for
152-
:envvar:`PYTHONCOERCECLOCALE`), and locale coercion is either disabled or
153-
fails. In such legacy locales, the interpreter will default to enabling UTF-8
154-
mode unless explicitly instructed not to do so.
155-
156-
The Python UTF-8 Mode can only be enabled at the Python startup. Its value
148+
The :ref:`Python UTF-8 Mode <utf8-mode>` is enabled by default.
149+
It can be disabled using the :option:`-X utf8=0 <-X>` command line
150+
option or the :envvar:`PYTHONUTF8=0 <PYTHONUTF8>` environment variable.
151+
The Python UTF-8 Mode can only be disabled at Python startup. Its value
157152
can be read from :data:`sys.flags.utf8_mode <sys.flags>`.
158153

154+
If the UTF-8 mode is disabled, the interpreter defaults to using
155+
the current locale settings, *unless* the current locale is identified
156+
as a legacy ASCII-based locale (as described for :envvar:`PYTHONCOERCECLOCALE`),
157+
and locale coercion is either disabled or fails.
158+
In such legacy locales, the interpreter will default to enabling UTF-8 mode
159+
unless explicitly instructed not to do so.
160+
159161
See also the :ref:`UTF-8 mode on Windows <win-utf8-mode>`
160162
and the :term:`filesystem encoding and error handler`.
161163

162-
.. seealso::
163-
164-
:pep:`686`
165-
Python 3.15 will make :ref:`utf8-mode` default.
166-
167164

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.. _os-procinfo:
169166

‎Doc/using/windows.rst

Lines changed: 17 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -1006,6 +1006,9 @@ UTF-8 mode
10061006
==========
10071007

10081008
.. versionadded:: 3.7
1009+
.. versionchanged:: next
1010+
1011+
Python UTF-8 mode is now enabled by default (:pep:`686`).
10091012

10101013
Windows still uses legacy encodings for the system encoding (the ANSI Code
10111014
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.
10141017
This may cause issues because UTF-8 is widely used on the internet
10151018
and most Unix systems, including WSL (Windows Subsystem for Linux).
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1017-
You can use the :ref:`Python UTF-8 Mode <utf8-mode>` to change the default text
1018-
encoding to UTF-8. You can enable the :ref:`Python UTF-8 Mode <utf8-mode>` via
1019-
the ``-X utf8`` command line option, or the ``PYTHONUTF8=1`` environment
1020-
variable. See :envvar:`PYTHONUTF8` for enabling UTF-8 mode, and
1021-
:ref:`setting-envvars` for how to modify environment variables.
1022-
1023-
When the :ref:`Python UTF-8 Mode <utf8-mode>` is enabled, you can still use the
1020+
The :ref:`Python UTF-8 Mode <utf8-mode>`, enabled by default, can help by
1021+
changing the default text encoding to UTF-8.
1022+
When the :ref:`UTF-8 mode <utf8-mode>` is enabled, you can still use the
10241023
system encoding (the ANSI Code Page) via the "mbcs" codec.
10251024

1026-
Note that adding ``PYTHONUTF8=1`` to the default environment variables
1027-
will affect all Python 3.7+ applications on your system.
1028-
If you have any Python 3.7+ applications which rely on the legacy
1029-
system encoding, it is recommended to set the environment variable
1030-
temporarily or use the ``-X utf8`` command line option.
1025+
You can disable the :ref:`Python UTF-8 Mode <utf8-mode>` via
1026+
the ``-X utf8=0`` command line option, or the ``PYTHONUTF8=0`` environment
1027+
variable. See :envvar:`PYTHONUTF8` for disabling UTF-8 mode, and
1028+
:ref:`setting-envvars` for how to modify environment variables.
1029+
1030+
.. hint::
1031+
Adding ``PYTHONUTF8={0,1}`` to the default environment variables
1032+
will affect all Python 3.7+ applications on your system.
1033+
If you have any Python 3.7+ applications which rely on the legacy
1034+
system encoding, it is recommended to set the environment variable
1035+
temporarily or use the ``-X utf8`` command line option.
10311036

10321037
.. note::
10331038
Even when UTF-8 mode is disabled, Python uses UTF-8 by default

‎Doc/whatsnew/3.15.rst

Lines changed: 25 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -172,11 +172,35 @@ production systems where traditional profiling approaches would be too intrusive
172172
Other language changes
173173
======================
174174

175+
* Python now uses UTF-8_ as the default encoding, independent of the system's
176+
environment. This means that I/O operations without an explicit encoding,
177+
e.g. ``open('flying-circus.txt')``, will use UTF-8.
178+
UTF-8 is a widely-supported Unicode_ character encoding that has become a
179+
*de facto* standard for representing text, including nearly every webpage
180+
on the internet, many common file formats, programming languages, and more.
181+
182+
This only applies when no ``encoding`` argument is given. For best
183+
compatibility between versions of Python, ensure that an explicit ``encoding``
184+
argument is always provided. The :ref:`opt-in encoding warning <io-encoding-warning>`
185+
can be used to identify code that may be affected by this change.
186+
The special special ``encoding='locale'`` argument uses the current locale
187+
encoding, and has been supported since Python 3.10.
188+
189+
To retain the previous behaviour, Python's UTF-8 mode may be disabled with
190+
the :envvar:`PYTHONUTF8=0 <PYTHONUTF8>` environment variable or the
191+
:option:`-X utf8=0 <-X>` command line option.
192+
193+
.. seealso:: :pep:`686` for further details.
194+
195+
.. _UTF-8: https://en.wikipedia.org/wiki/UTF-8
196+
.. _Unicode: https://home.unicode.org/
197+
198+
(Contributed by Adam Turner in :gh:`133711`; PEP 686 written by Inada Naoki.)
199+
175200
* Several error messages incorrectly using the term "argument" have been corrected.
176201
(Contributed by Stan Ulbrych in :gh:`133382`.)
177202

178203

179-
180204
New modules
181205
===========
182206

‎Include/cpython/initconfig.h

Lines changed: 6 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -102,15 +102,14 @@ typedef struct PyPreConfig {
102102

103103
/* Enable UTF-8 mode? (PEP 540)
104104
105-
Disabled by default (equals to 0).
105+
If equal to 1, use the UTF-8 encoding and use "surrogateescape" for the
106+
stdin & stdout error handlers.
106107
107-
Set to 1 by "-X utf8" and "-X utf8=1" command line options.
108-
Set to 1 by PYTHONUTF8=1 environment variable.
108+
Enabled by default (equal to 1; PEP 686), or if Py_UTF8Mode=1,
109+
or if "-X utf8=1" or PYTHONUTF8=1.
109110
110-
Set to 0 by "-X utf8=0" and PYTHONUTF8=0.
111-
112-
If equals to -1, it is set to 1 if the LC_CTYPE locale is "C" or
113-
"POSIX", otherwise it is set to 0. Inherit Py_UTF8Mode value value. */
111+
Set to 0 by "-X utf8=0" or PYTHONUTF8=0.
112+
*/
114113
int utf8_mode;
115114

116115
/* If non-zero, enable the Python Development Mode.

‎Lib/locale.py

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -651,7 +651,8 @@ def getpreferredencoding(do_setlocale=True):
651651
if sys.flags.warn_default_encoding:
652652
import warnings
653653
warnings.warn(
654-
"UTF-8 Mode affects locale.getpreferredencoding(). Consider locale.getencoding() instead.",
654+
"UTF-8 Mode affects locale.getpreferredencoding(). "
655+
"Consider locale.getencoding() instead.",
655656
EncodingWarning, 2)
656657
if sys.flags.utf8_mode:
657658
return 'utf-8'

‎Lib/subprocess.py

Lines changed: 1 addition & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -380,8 +380,7 @@ def _text_encoding():
380380

381381
if sys.flags.utf8_mode:
382382
return "utf-8"
383-
else:
384-
return locale.getencoding()
383+
return locale.getencoding()
385384

386385

387386
def call(*popenargs, timeout=None, **kwargs):

‎Lib/test/test_cmd_line.py

Lines changed: 6 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -300,6 +300,10 @@ def run_utf8_mode(arg):
300300
cmd = [sys.executable, '-X', 'utf8', '-c', code, arg]
301301
return subprocess.run(cmd, stdout=subprocess.PIPE, text=True)
302302

303+
def run_no_utf8_mode(arg):
304+
cmd = [sys.executable, '-X', 'utf8=0', '-c', code, arg]
305+
return subprocess.run(cmd, stdout=subprocess.PIPE, text=True)
306+
303307
valid_utf8 = 'e:\xe9, euro:\u20ac, non-bmp:\U0010ffff'.encode('utf-8')
304308
# invalid UTF-8 byte sequences with a valid UTF-8 sequence
305309
# in the middle.
@@ -312,7 +316,8 @@ def run_utf8_mode(arg):
312316
)
313317
test_args = [valid_utf8, invalid_utf8]
314318

315-
for run_cmd in (run_default, run_c_locale, run_utf8_mode):
319+
for run_cmd in (run_default, run_c_locale, run_utf8_mode,
320+
run_no_utf8_mode):
316321
with self.subTest(run_cmd=run_cmd):
317322
for arg in test_args:
318323
proc = run_cmd(arg)

‎Lib/test/test_embed.py

Lines changed: 2 additions & 8 deletions
Original file line numberDiff line numberDiff line change
@@ -543,7 +543,7 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
543543
'configure_locale': True,
544544
'coerce_c_locale': False,
545545
'coerce_c_locale_warn': False,
546-
'utf8_mode': False,
546+
'utf8_mode': True,
547547
}
548548
if MS_WINDOWS:
549549
PRE_CONFIG_COMPAT.update({
@@ -560,7 +560,7 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
560560
configure_locale=False,
561561
isolated=True,
562562
use_environment=False,
563-
utf8_mode=False,
563+
utf8_mode=True,
564564
dev_mode=False,
565565
coerce_c_locale=False,
566566
)
@@ -805,12 +805,6 @@ def get_expected_config(self, expected_preconfig, expected,
805805
'stdio_encoding', 'stdio_errors'):
806806
expected[key] = self.IGNORE_CONFIG
807807

808-
if not expected_preconfig['configure_locale']:
809-
# UTF-8 Mode depends on the locale. There is no easy way
810-
# to guess if UTF-8 Mode will be enabled or not if the locale
811-
# is not configured.
812-
expected_preconfig['utf8_mode'] = self.IGNORE_CONFIG
813-
814808
if expected_preconfig['utf8_mode'] == 1:
815809
if expected['filesystem_encoding'] is self.GET_DEFAULT_CONFIG:
816810
expected['filesystem_encoding'] = 'utf-8'

‎Lib/test/test_utf8_mode.py

Lines changed: 3 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -89,8 +89,8 @@ def test_env_var(self):
8989
# the UTF-8 mode
9090
if not self.posix_locale():
9191
# PYTHONUTF8 should be ignored if -E is used
92-
out = self.get_output('-E', '-c', code, PYTHONUTF8='1')
93-
self.assertEqual(out, '0')
92+
out = self.get_output('-E', '-c', code, PYTHONUTF8='0')
93+
self.assertEqual(out, '1')
9494

9595
# invalid mode
9696
out = self.get_output('-c', code, PYTHONUTF8='xxx', failure=True)
@@ -116,7 +116,7 @@ def test_filesystemencoding(self):
116116
# PYTHONLEGACYWINDOWSFSENCODING disables the UTF-8 mode
117117
# and has the priority over -X utf8 and PYTHONUTF8
118118
out = self.get_output('-X', 'utf8', '-c', code,
119-
PYTHONUTF8='strict',
119+
PYTHONUTF8='xxx',
120120
PYTHONLEGACYWINDOWSFSENCODING='1')
121121
self.assertEqual(out, 'mbcs/replace')
122122

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