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Deprecation of locale.getdefaultlocale breaks POSIX compatibility on Windows platform #130796

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Deprecation of locale.getdefaultlocale breaks POSIX compatibility on Windows platform#130796
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3.15bugs and security fixespendingThe issue will be closed if no feedback is providedrelease-blockerstdlibStandard Library Python modules in the Lib/ directorytype-featureA feature request or enhancement

Description

@Wedge009
Wedge009
opened on Mar 3, 2025

Bug report

Bug description:

Follow-up to #90817.

In summary, without getdetfaultlocale, tools that rely on POSIX locales cannot reliably get the same information from getlocale or setlocale on Windows.

For example, Linux will return something like 'en_AU'. But on Windows, getlocale will return something like 'English_Australia'.

CPython versions tested on:

3.12, 3.13

Operating systems tested on:

Windows, Linux

Linked PRs

Activity

  1. Wedge009
    added
    type-bugAn unexpected behavior, bug, or error
    on Mar 3, 2025
  2. picnixz
    added
    stdlibStandard Library Python modules in the Lib/ directory
    on Mar 3, 2025
  3. picnixz
    added this to Locale issues 🗺on Mar 3, 2025
  4. github-project-automation
    moved this to Todo in Locale issues 🗺on Mar 3, 2025
  5. picnixz
    added
    type-featureA feature request or enhancement
    and removed
    type-bugAn unexpected behavior, bug, or error
    on Mar 3, 2025

  6. picnixz commented on Mar 3, 2025

    @picnixz
    picnixz
    on Mar 3, 2025
    Member

    I'm afraid that this issue is too late because the ship has already sailed. I don't remember well which alternatives there are for that kind of issue so I'll tag @vstinner who was responsible for the original patch. However, I doubt that we would reinstate locale.getdefaultlocale (or we would do something else).

    EDIT: Actually it seems to only be deprecated and planned for removal in 3.15: #111187.

  7. picnixz
    added
    pendingThe issue will be closed if no feedback is provided
    on Mar 3, 2025

  8. Wedge009 commented on Mar 3, 2025

    @Wedge009
    Wedge009
    on Mar 3, 2025
    Author

    I don't think I - or others - was asking for a reinstatement, just a viable alternative for the cases where POSIX locales are expected for use with other tools such as gettext.

    And yes, removal is planned for 3.15, hence the discussion now. As noted in wesnoth/wesnoth#9972 (comment), it's difficult to know how to work around the removal of getdefaultlocale when we don't know what future plans are - if any - for any replacement functionality.

  9. vstinner commented on Mar 3, 2025

    @vstinner
    vstinner
    on Mar 3, 2025
    Member

    On my Windows 11 VM, getlocale() returns a "POSIX" locale as expected:

    vstinner@WIN C:\victor\python\main>python
Python 3.14.0a5+ (heads/pr/130452-dirty:6a5dc1a8ba6, Feb 26 2025, 17:30:51) [MSC v.1943 32 bit (Intel)] on win32
>>> import locale
>>> locale.getlocale()
('fr_FR', 'cp1252')
>>> locale.setlocale(locale.LC_CTYPE, None)
'French_France.1252'

    Can you please elaborate your issue? What are the results of locale.getlocale() and locale.setlocale(locale.LC_CTYPE, None)?

  10. Wedge009 commented on Mar 3, 2025

    @Wedge009
    Wedge009
    on Mar 3, 2025
    Author

    It's as I described above and in #90817 (comment).

    Python 3.13.2 (tags/v3.13.2:4f8bb39, Feb  4 2025, 15:23:48) [MSC v.1942 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
import locale
locale.getlocale()
('English_Australia', '1252')
locale.setlocale(locale.LC_CTYPE)
'English_Australia.1252'
locale.getdefaultlocale()

Warning (from warnings module):
  File "<pyshell#3>", line 1
DeprecationWarning: 'locale.getdefaultlocale' is deprecated and slated for removal in Python 3.15. Use setlocale(), getencoding() and getlocale() instead.
('en_AU', 'cp1252')

    @shineworld reported something similar in #90817 (comment)

    @Elvish-Hunter reports the same in wesnoth/wesnoth#9972 (comment)

  11. 2trvl commented on Mar 6, 2025

    @2trvl
    2trvl
    on Mar 6, 2025
    Contributor

    The reason why POSIX compatibility breaks is this:

    The windows_locale dictionary is only used by the locale.getdefaultlocale() function and this function is deprecated: it will be removed in Python 3.15.

    Windows 22H2, Python 3.13.1:

    >>> locale.setlocale(locale.LC_CTYPE)
'English_United States.1252'

>>> locale.getlocale()
('English_United States', '1252')

>>> locale.getdefaultlocale()
('en_US', 'cp1252')

  12. 2trvl commented on Mar 7, 2025

    @2trvl
    2trvl
    on Mar 7, 2025
    Contributor

    I would like to know how can I find out the system locale for LC_TIME without using defaultlocale?

    >>> locale.getdefaultlocale(["LC_TIME"])
('ru_RU', 'UTF-8')

>>> locale.setlocale(locale.LC_TIME)
'C'

>>> locale.getlocale(locale.LC_TIME)
(None, None)

    Apparently I have no other option than to change the interpreter locale:

    >>> locale.setlocale(locale.LC_TIME, "")
'ru_RU.UTF-8'

>>> locale.getlocale(locale.LC_TIME)
('ru_RU', 'UTF-8')

    It is generally a bad idea to call setlocale() in some library routine, since as a side effect it affects the entire program. Saving and restoring it is almost as bad: it is expensive and affects other threads that happen to run before the settings have been restored.

    I see that the original reason for removal is the use of non-ASCII characters in locale. Do you think this is a common case?

    If fixing the problem is difficult or impossible, perhaps it is worth introducing a similar function called getpreferredlocale?

  13. nagadomi
    added a commit that references this issue on May 10, 2025

    iw3: waifu2x: Use short language name on Windows in GUI

    78ea6b6

  14. StanFromIreland commented on May 17, 2025

    @StanFromIreland
    StanFromIreland
    on May 17, 2025
    Member

    The windows_locale dictionary is well out of date anyway (and is no longer updated from what I can tell), so keeping the function is not great either. The dictionary itself is not deprecated, and so you should be able to use it. If we do want to provide this functionality on windows, it should be better though out.

    I was unable to find any windows API that returns the POSIX format.

  15. 6 remaining items

  16. github-project-automation
    moved this to Todo in Release and Deferred blockers 🚫on Dec 20, 2025

  17. gpshead commented on Dec 20, 2025

    @gpshead
    gpshead
    on Dec 20, 2025
    Member

    I added the 3.15 release blocker labels as if we don't solve this for 3.15 we should stop the deprecation.

  18. chris-eibl commented on Dec 22, 2025

    @chris-eibl
    chris-eibl
    on Dec 22, 2025
    Member

    Related: #82986 and #67613 (where @eryksun presented a rough prototype to auto-populate locale_alias, #67613 (comment)).

  19. vstinner
    added a commit that references this issue on Dec 22, 2025

    pythongh-130796: Undeprecate locale.getdefaultlocale()

    499596c

  20. vstinner commented on Dec 22, 2025

    @vstinner
    vstinner
    on Dec 22, 2025
    Member

    I wasn't aware that locale.getdefaultlocale() is the only way to get a POSIX-like locale name on Windows when I deprecated the function. I didn't know neither that there are users of the "read the locale from environment variables" feature. There is no good replacement for these features, so I wrote #143069 to undeprecate locale.getdefaultlocale().

  21. vstinner commented on Dec 22, 2025

    @vstinner
    vstinner
    on Dec 22, 2025
    Member

    FYI getdefaultlocale() was not removed yet because we are aware of these issues.

  22. malemburg commented on Dec 22, 2025

    @malemburg
    malemburg
    on Dec 22, 2025
    Member

    Whereas locale.getlocale() internally calls C setlocale(LC_CTYPE, NULL) which returns German_Austria for me.

    This is then further processed via locale.normalize() which does a lookup in locale_alias

    cpython/Lib/locale.py

    Line 763 in 786f464

    The locale_alias table maps lowercase alias names to C locale names

    which fails for German_Austria and all the other examples given in this issue, but succeeds for e.g.

    'french_france':                        'fr_FR.ISO8859-1',
'german_germany':                       'de_DE.ISO8859-1',

    Thus, @vstinner gets fr_FR above and @malemburg gets de_DE in #90817 (comment).

    I suppose we could make locale.getlocale() work for this use case by adding the Windows locale names to the locale_alias table.

    Apparently I have no other option than to change the interpreter locale:

    locale.setlocale(locale.LC_TIME, "")
    'ru_RU.UTF-8'

    locale.getlocale(locale.LC_TIME)
    ('ru_RU', 'UTF-8')

    This is why I had added locale.getdefaultlocale() a long while ago: I did not want to break libraries which rely on other custom local settings by calling setlocale(LC_ALL, ""), since this then applies to the whole process. Doing so at random times can be especially problematic for embedded Python interpreters.

    Nowadays, CPython does call setlocale(LC_CTYPE, NULL); during startup and that was what triggered the deprecation of locale.setdefaultlocale(). However, you are right in that the call only configures the LC_CTYPE part of the locale settings, not the other ones, such as LC_TIME. We should probably change the startup code to call setlocale(LC_ALL, "") instead - as recommended by the C lib docs.

    So I guess two changes would be necessary to fix this situation:

    • add the Windows locale names to the alias table

    • have Python init all locale parts during startup

    or we leave locale.getdefaultlocale() around, but given that it's purpose has been superseded, I don't think that's a good idea.

  23. vstinner commented on Dec 23, 2025

    @vstinner
    vstinner
    on Dec 23, 2025
    Member

    setlocale(LC_CTYPE, "") is called by Python during startup: in _Py_SetLocaleFromEnv(LC_CTYPE) which is called by _PyPreConfig_Write().

  24. vstinner
    added a commit that references this issue on Dec 23, 2025

    gh-130796: Undeprecate locale.getdefaultlocale() (#143069)

    6536fab

  25. vstinner commented on Dec 23, 2025

    @vstinner
    vstinner
    on Dec 23, 2025
    Member

    I close the issue. The function is no longer deprecated in Python 3.15.

  26. vstinner
    closed this as completedon Dec 23, 2025
  27. github-project-automation
    moved this from Todo to Done in Locale issues 🗺on Dec 23, 2025
  28. github-project-automation
    moved this from Todo to Done in Release and Deferred blockers 🚫on Dec 23, 2025
  29. hugovk
    added 2 commits that reference this issue on Jun 11, 2026

    [3.14] gh-130796: Undeprecate locale.getdefaultlocale() (GH-143069) (#…

    58c6e78

    [3.13] gh-130796: Undeprecate locale.getdefaultlocale() (GH-143069) (#…

    06dce35
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