Recording and restoring the LC_NUMERIC locale setting with LC_ALL=C.UTF-8 no longer works with Python 3.13.3

[bouweandela]

Bug report

Bug description:

We have project where we record the current value of locale.LC_NUMERIC, change it to a different value to use a library that needs a particular locale setting, and then restore it to the original value. This stopped working with Python 3.13.3. I suspect this may be a bug, but please correct me if I misunderstood the functionality.

Example with Python 3.13.3:

$ docker run -it python:3.13.3 
Python 3.13.3 (main, May  9 2025, 23:49:05) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os, locale
>>> os.environ
environ({'PATH': '/usr/local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin', 'HOSTNAME': '2e43b79a7135', 'TERM': 'xterm', 'GPG_KEY': '7169605F62C751356D054A26A821E680E5FA6305', 'PYTHON_VERSION': '3.13.3', 'PYTHON_SHA256': '40f868bcbdeb8149a3149580bb9bfd407b3321cd48f0be631af955ac92c0e041', 'HOME': '/root', 'LC_CTYPE': 'C.UTF-8'})
>>> os.environ["LC_ALL"] = "C.UTF-8"
>>> locale.setlocale(locale.LC_ALL, "")
'C.UTF-8'
>>> locale.getlocale(locale.LC_NUMERIC)
('en_US', 'UTF-8')
>>> locale.setlocale(locale.LC_NUMERIC, locale.getlocale(locale.LC_NUMERIC))
Traceback (most recent call last):
  File "<python-input-5>", line 1, in <module>
    locale.setlocale(locale.LC_NUMERIC, locale.getlocale(locale.LC_NUMERIC))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.13/locale.py", line 615, in setlocale
    return _setlocale(category, locale)
locale.Error: unsupported locale setting

The same example works fine with earlier versions of Python, for example 3.13.2:

$ docker run -it python:3.13.2 
Python 3.13.2 (main, Apr  8 2025, 04:27:11) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os, locale
>>> os.environ
environ({'PATH': '/usr/local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin', 'HOSTNAME': '64800af765e4', 'TERM': 'xterm', 'GPG_KEY': '7169605F62C751356D054A26A821E680E5FA6305', 'PYTHON_VERSION': '3.13.2', 'PYTHON_SHA256': 'd984bcc57cd67caab26f7def42e523b1c015bbc5dc07836cf4f0b63fa159eb56', 'HOME': '/root', 'LC_CTYPE': 'C.UTF-8'})
>>> os.environ["LC_ALL"] = "C.UTF-8"
>>> locale.setlocale(locale.LC_ALL, "")
'C.UTF-8'
>>> locale.getlocale(locale.LC_NUMERIC)
('C', 'UTF-8')
>>> locale.setlocale(locale.LC_NUMERIC, locale.getlocale(locale.LC_NUMERIC))
'C.UTF-8'

It looks like this may have been caused by the changes in #129647.

CPython versions tested on:

3.13

Operating systems tested on:

No response

Linked PRs

• gh-133967: Do not normalize locale name 'C.UTF-8' to 'en_US.UTF-8' #135347
• [3.14] gh-133967: Do not normalize locale name 'C.UTF-8' to 'en_US.UTF-8' (GH-135347) #135349
• [3.13] gh-133967: Do not normalize locale name 'C.UTF-8' to 'en_US.UTF-8' (GH-135347) #135350

[mvkashyap]

I'm at the CPython sprints right now, so I can take a look at this.

[mvkashyap]

While I'm unable to reproduce your exact issue, I'm debugging a slightly similar issue that I'm having on my machine. Instead of the error, I'm getting this:

Type "help", "copyright", "credits" or "license" for more information.
>>> import os, locale
>>> os.environ
environ({'PATH': '/usr/local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin', 'HOSTNAME': '64800af765e4', 'TERM': 'xterm', 'GPG_KEY': '7169605F62C751356D054A26A821E680E5FA6305', 'PYTHON_VERSION': '3.13.2', 'PYTHON_SHA256': 'd984bcc57cd67caab26f7def42e523b1c015bbc5dc07836cf4f0b63fa159eb56', 'HOME': '/root', 'LC_CTYPE': 'C.UTF-8'})
>>> os.environ["LC_ALL"] = "C.UTF-8"
>>> locale.setlocale(locale.LC_ALL, "")
'C.UTF-8'
>>> locale.getlocale(locale.LC_NUMERIC)
('en_US, UTF-8)
>>> locale.setlocale(locale.LC_NUMERIC, ('C', 'UTF-8'))
'en_US.UTF-8'

[bouweandela]

Thank you for taking the time to investigate! Are you using the Python containers from Dockerhub? In my tests with those containers, the problem only appears with Python 3.13.3, while for you, it seems to partially appear in 3.13.2, but without the crash when trying to set locale.LC_NUMERIC. Could you try if you can reproduce it with Python 3.13.3?

[mvkashyap]

Ah, this is for 3.13.3!

[valeriupredoi]

many thanks @mvkashyap 🍺

[serhiy-storchaka]

This was caused by #129646.

The locale.aliases file from X.org distribution (https://cgit.freedesktop.org/xorg/lib/libX11/tree/nls/locale.alias.pre) maps locale C.utf8 to en_US.UTF-8. The problem is that en_US.UTF-8 may not exist on all platforms, especially on the old ones. This is a congenital defect in the locale module design -- that it uses a static platform-independent mapping.

cc @malemburg

[malemburg]

I think the problem is different: the en_US locale files may not have been generated on the base system, whereas the C locale should always be present.

In any case, we should probably special case the C local mappings in the script Tools/i18n/makelocalealias.py to not have them alias to en_US (even though that is what they normally refer to).

[malemburg]

BTW: I do wonder why the locale files don't exist in the official Python docker image.

Note that locale.getlocale()returns ('en_US', 'UTF-8') in the 3.13.3 image, but the /usr/share/locale is missing the en_US subdir. In the 3.12.3 image, it returns ('C', 'UTF-8'), which is present. This image is also missing the en_US subdir.

This suggests that the official images are somewhat broken... where can this be reported ?

[malemburg]

Checking using locale -a, only the locales C, C.utf8 and POSIX are available in those images, so I guess they are based on a minimal installation of Debian.