Make encoding="locale" uses locale encoding even in UTF-8 mode is enabled.

[methane]

BPO	47000
Nosy	@malemburg, @vstinner, @ezio-melotti, @methane, @eryksun
PRs	bpo-47000: Make io.text_encoding() respects UTF-8 mode #32003 bpo-47000: Add locale.getencoding() #32068

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-03-13.06:09:52.037>
labels = ['expert-unicode', '3.11']
title = 'Make encoding="locale" uses locale encoding even in UTF-8 mode is enabled.'
updated_at = <Date 2022-04-04.02:47:05.537>
user = 'https://github.com/methane'

bugs.python.org fields:

activity = <Date 2022-04-04.02:47:05.537>
actor = 'methane'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Unicode']
creation = <Date 2022-03-13.06:09:52.037>
creator = 'methane'
dependencies = []
files = []
hgrepos = []
issue_num = 47000
keywords = ['patch']
message_count = 16.0
messages = ['415028', '415118', '415146', '415147', '415219', '415240', '415767', '415768', '415769', '415851', '415867', '415922', '416329', '416330', '416332', '416649']
nosy_count = 5.0
nosy_names = ['lemburg', 'vstinner', 'ezio.melotti', 'methane', 'eryksun']
pr_nums = ['32003', '32068']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue47000'
versions = ['Python 3.11']

Linked PRs

• gh-91156: Document how TextIOWrapper interacts with UTF-8 mode #132885
• [3.13] gh-91156: Document how TextIOWrapper interacts with UTF-8 mode (GH-132885) #133323

[methane]

Currently, encoding="locale" is just shortcut of encoding=locale.getpreferredencoding(False).

encoding="locale" means that "locale encoding should be used here, even if Python default encoding is changed to UTF-8".

I am not sure that UTF-8 mode becomes the default or not.
But some user want to use UTF-8 mode to change default encoding in their Python environments without waiting Python default encoding changed.

So I think encoding="locale" should use real locale encoding (ACP on Windows) regardless UTF-8 mode is enabled or not.

Currently, UTF-8 mode affects to _Py_GetLocaleEncoding(). So it is difficult that make encoding="locale" ignores UTF-8 mode.
Is it safe to use locale.getlocale(locale.LC_CTYPE)[1] or "UTF-8"?

[methane]

I created a related topic on discuss.python.org.
https://discuss.python.org/t/jep-400-utf-8-by-default-and-future-of-python/14246

If we recommend PYTHONUTF8 as opt-in "UTF-8 by default", encoding="locale" should locale encoding in UTF-8 mode.

If we don't change PYTHONUTF8 behavior, we need yet another option for opt-in "UTF-8 by default".

[vstinner]

There are multiple "locale encodings":

• "current" locale encoding: locale.nl_langinfo(locale.CODESET)
• "Python" locale encoding: locale.getpreferredencoding(False), ignore the locale in UTF-8 Mode (always return "UTF-8"), ignore the locale on Android and VxWorks (always return "UTF-8")
• Python "filesystem" encoding: similar to the Python locale encoding, but always use UTF-8 on Android, macOS and VxWorks

Include/pyport.h:
---

#if defined(__ANDROID__) || defined(__VXWORKS__)
   // Use UTF-8 as the locale encoding, ignore the LC_CTYPE locale.
   // See _Py_GetLocaleEncoding(), PyUnicode_DecodeLocale()
   // and PyUnicode_EncodeLocale().
#  define _Py_FORCE_UTF8_LOCALE
#endif

#if defined(_Py_FORCE_UTF8_LOCALE) || defined(__APPLE__)
   // Use UTF-8 as the filesystem encoding.
   // See PyUnicode_DecodeFSDefaultAndSize(), PyUnicode_EncodeFSDefault(),
   // Py_DecodeLocale() and Py_EncodeLocale().
#  define _Py_FORCE_UTF8_FS_ENCODING
#endif

---

See bpo-43552 "Add locale.get_locale_encoding() and locale.get_current_locale_encoding()" (rejected).

Marc-Andre Lemburg dislikes locale.getpreferredencoding(False) API and suggested adding a new function locale.getencoding() with no argument:
https://bugs.python.org/issue46659#msg412667

[vstinner]

So I think encoding="locale" should use real locale encoding (ACP on Windows) regardless UTF-8 mode is enabled or not.

If you want to change the default, would it be possible to add a function to get this encoding?

[methane]

I created another topic relating this issue.
https://discuss.python.org/t/add-legacy-text-encoding-option-to-make-utf-8-default/14281

If we add another option (e.g. legacy_text_encoding), we do not need to change UTF-8 mode behavior.

[malemburg]

FWIW: I don't think the "locale" encoding is a good idea. Instead of
trying to fix this to make it more usable, I'd suggest to deprecate
and remove it again.

When it comes to encodings, explicit is better than implicit.

If an application wants to work with some user defined locale settings,
it's better for the application to decide where to pick the locale
settings from, e.g. the OS, the UI, an application config setting,
etc.

There are too many ways this can be done and trying to build
magic to determine the "right" one is bound to fail in one way or
another.

[vstinner]

Is it safe to use locale.getlocale(locale.LC_CTYPE)[1] or "UTF-8"?

I would like to deprecate getlocale(), see bpo-43557.

[vstinner]

But some user want to use UTF-8 mode to change default encoding in their Python environments without waiting Python default encoding changed.

IMO it's a different use case and it should be a different thing. Changing encoding="locale" today is too late, since it's already shipped in Python 3.10 (PEP-597).

I proposed the "current locale" name to distinguish it from the existing "locale":

• "current locale": LC_CTYPE locale encoding or ANSI code page
• "locale": "UTF-8" in UTF-8 Mode, or the current locale

The unclear part to me is if "current locale" must change if the LC_CTYPE locale is changed, or if it should be read once at startup and then never change.

There *are* use case to really read the *current* LC_CTYPE locale encoding. There is already C API for that:

• PyUnicode_EncodeLocale()
• PyUnicode_DecodeLocale(), PyUnicode_DecodeLocaleAndSize()

See also the "current_locale" parameter of the private API _Py_EncodeLocaleEx() and _Py_DecodeLocaleEx().

[vstinner]

I propose:

• sys.getfilesystemencoding(): Python filesystem encoding, return "UTF-8" if the Python UTF-8 Mode is enabled

  • Implementation: PyConfig.filesystem_encoding
  • https://docs.python.org/dev/library/sys.html#sys.getfilesystemencoding
  • https://docs.python.org/dev/glossary.html#term-filesystem-encoding-and-error-handler
  • https://docs.python.org/dev/c-api/init_config.html#c.PyConfig.filesystem_encoding

• locale.getencoding(): Get the locale encoding, LC_CTYPE locale encoding or the Windows ANSI code page, *read at Python startup*. Ignore the Python UTF-8 Mode.

  • https://docs.python.org/dev/glossary.html#term-locale-encoding
  • Implementation: _Py_GetLocaleEncoding()
  • Existing *private* function: _locale._get_locale_encoding()

• locale.getencoding(current=True): Get the *current* locale encoding. The difference with locale.getencoding() is that on Unix, it gets the LC_CTYPE locale encoding at each call.

  • Implementation: _Py_GetLocaleEncoding() modified to ignore the UTF-8 mode.

None of these functions do locale.setlocale(locale.LC_CTYPE, "") to get the user preferred encoding.

Only the locale.getpreferredencoding() function uses locale.setlocale(locale.LC_CTYPE, "").

Usage of locale.getpreferredencoding() should be discouraged in the documentation, but I don't think that it can be deprecated and scheduled for removal right now: too much code rely on it :-(

---

So we have 3 encodings:

• Python filesystem encoding
• Locale encoding
• Current locale encoding

Examples of usage:

• Python filesystem encoding:

  • os.fsdecode() / os.fsencode()
  • C: PyUnicode_EncodeFSDefault() / PyUnicode_DecodeFSDefault()

• Locale encoding

  • _locale._get_locale_encoding()
  • On Unix, os.device_encoding()
  • To initialize PyConfig.stdio_encoding and PyConfig.filesystem_encoding

• Current locale encoding

  • PyUnicode_EncodeLocale() / PyUnicode_DecodeLocale()
  • "current_locale" parameter of private _Py_EncodeLocaleEx() / _Py_DecodeLocaleEx()

[methane]

• sys.getfilesystemencoding(): Python filesystem encoding, return "UTF-8" if the Python UTF-8 Mode is enabled

Yes, althoguh PYTHONLEGACYWINDOWSFSENCODING takes priority.

• locale.getencoding(): Get the locale encoding, LC_CTYPE locale encoding or the Windows ANSI code page, *read at Python startup*. Ignore the Python UTF-8 Mode.

I proposed locale.get_encoding() in the PEP-686. I will remove underscore if you don't like it.

• locale.getencoding(current=True): Get the *current* locale encoding. The difference with locale.getencoding() is that on Unix, it gets the LC_CTYPE locale encoding at each call.

Hmm, I don't add it to the PEP-686 because it is not relating to UTF-8 mode nor EncodingWarning.

Since locale.getencoding() returns locale encoding on startup, how about this idea?

• sys.getlocaleencoding() -- Get the locale encoding read at Python startup.
• locale.getencoding() -- Get the current locale encoding.

Note that we have sys.getdefaultencoding() and sys.getfilesystemencoding(). sys.getlocaleencoding() looks consistent with them.

[vstinner]

sys.getlocaleencoding() versus locale.getencoding().

For me, the Python locale module should use the C API to access the Unix locales like LC_CTYPE, nl_langinfo(CODESET), etc.

The sys module are more for things specific to Python, like sys.getfilesystemencoding().

Since sys.getlocaleencoding() would be a fixed value for the whole process life time, I agree that the sys module is a better place.

I can write a PR adding sys.getlocaleencoding() if we agree on the API.

[methane]

I am not sure about we really need "locale encoding at Python startup".

For this issue, I don't want to change encoding="locale" behavior except ignore UTF-8 mode. So what I want is "current locale encoding" or
ANSI codepage on Windows.

On the other hand, I know Eryk wants to support locale on Windows. So locale.get_encoding() might return current locale encoding (not ANSI codepage) even on Windows.
If so, I will use sys.getlocaleencoding() to implement encoding="locale" to keep using ANSI codepage, instead of adding yet another "get locale encoding" function.