[Bug]: Current locale not honored on Windows

Bug summary

Date formatting using strftime format codes should use the current locale's names. While this works as expected on Linux, ConciseDateFormatter by default shows English month names etc. when running on a non-English locale. It only works correctly after explicitly setting the locale to the current one by locale.setlocale(locale.LC_ALL, '').
Setting axes.formatter.use_locale to True doesn't have any effect on Windows unless you explicitly set the locale to the current one by locale.setlocale(locale.LC_ALL, '') (again, it works correctly on Linux). I don't understand why this is so as __init__ does exactly the same thing but for some reason without effect on Windows:

matplotlib/lib/matplotlib/__init__.py

Lines 972 to 973 in f5ff5d7

if rcParams['axes.formatter.use_locale']:

locale.setlocale(locale.LC_ALL, '')

Code for reproduction

import locale
import platform

import matplotlib as mpl
import matplotlib.pyplot as plt
import numpy as np

dates = np.arange(np.datetime64('2022-12-01'), np.datetime64('2022-12-02'), np.timedelta64(1, 'h'))
values = np.zeros_like(dates, dtype=int)

def plot(title):
    fig, ax = plt.subplots(figsize=(6, 1.5), layout='constrained')
    ax.plot(dates, values, 'white')
    ax.xaxis.set_major_formatter(mpl.dates.ConciseDateFormatter(mpl.dates.AutoDateLocator()))
    ax.text(.5, .5,
            f"{locale.getlocale() = }\n{ax.yaxis.get_major_formatter().get_useLocale() = }\n({platform.system()})",
            transform=ax.transAxes, ha='center', va='center')
    ax.set_title(title)
    fig.savefig(title + '.png')

# use \N{MODIFIER LETTER COLON} instead of regular colon for Windows file names
plot('1꞉ default')

with mpl.rc_context({'axes.formatter.use_locale': True}):
    plot('2꞉ axes.formatter.use_locale = True')

locale.setlocale(locale.LC_ALL, '')
plot("3꞉ setlocale(locale.LC_ALL, '')")

with mpl.rc_context({'axes.formatter.use_locale': True}):
    plot("4꞉ axes.formatter.use_locale = True after setlocale(locale.LC_ALL, '')")

Actual outcome

On Windows:

1꞉ default--> should be Dez instead of Dec

2: axes.formatter.use_locale = True --> should be 0,00 instead of 0.00

3: after setlocale(locale.LC_ALL, '') --> this works correctly

4: axes.formatter.use_locale = True after setlocale(locale.LC_ALL, '') --> this works correctly too

Expected outcome

On Linux

All four cases are correct. The expected outcome on Windows is the same as on Linux.

Additional information

The issue is only on Windows, everything is fine on Linux (can't test it on MacOS but I suppose it'd be the same as on Linux).

Operating system

Windows

Matplotlib Version

3.6.3

Matplotlib Backend

No response

Python version

3.10

Jupyter version

No response

Installation

conda

As a side note, the Formatting date ticks using ConciseDateFormatter example incorrectly speaks about localization where it must read customization: Python's strftime format codes always use the locale's names for days, months etc. so there is neither the necessity nor the possibility of a localization in matplotlib. If you want to localize the names you must set the locale accordingly, you can't do it by changing the formatter's formats.

@oscargus

I found the culprit: this has nothing to do with Windows or Linux but with the Gtk backend. Using any Gtk backend sets the locale to the user's default setting (same as locale.setlocale(locale.LC_ALL, '')).

Running

import subprocess
import matplotlib as mpl

for backend in mpl.rcsetup.all_backends:
    cp = subprocess.run(['python.exe', 'use_backend.py', backend], capture_output=True)
    print(cp.stdout.decode())

with the following use_backend.py:

import locale
import sys
import matplotlib as mpl
import matplotlib.pyplot as plt

backend = sys.argv[1]
try:
    print(backend, end=': ')
    old_locale = locale.setlocale(locale.LC_NUMERIC)
    mpl.use(backend)
    plt.subplots()
    new_locale = locale.setlocale(locale.LC_NUMERIC)
    print(f'left {old_locale} unchanged' if new_locale == old_locale else f'changed locale from {old_locale} to {new_locale}')
except Exception as ex:
    print(f'raised {type(ex).__name__}')

gives the following output (Windows 10):

GTK3Agg: changed locale from C to de_DE
GTK3Cairo: changed locale from C to de_DE
GTK4Agg: changed locale from C to de_DE
GTK4Cairo: changed locale from C to de_DE
MacOSX: raised ImportError
nbAgg: left C unchanged
QtAgg: left C unchanged
QtCairo: left C unchanged
Qt5Agg: left C unchanged
Qt5Cairo: left C unchanged
TkAgg: left C unchanged
TkCairo: left C unchanged
WebAgg: left C unchanged
WX: left C unchanged
WXAgg: left C unchanged
WXCairo: left C unchanged
agg: left C unchanged
cairo: left C unchanged
pdf: left C unchanged
pgf: left C unchanged
ps: left C unchanged
svg: left C unchanged
template: left C unchanged

@oscargus: could you remove the "OS: Microsoft" tag, please.

I don't have a Mac so maybe someone can run the above on MacOS, just for completeness.

Regarding using locale number and date formatting in matplotlib:

The rcParams['axes.formatter.use_locale'] parameter doesn't work as one would expect: it only has any effect (sets the locale) when set in the config file (see here); later changes directly to the mpl.rcParams or using the mpl.rc_context context manager have no effect as they don't trigger the setting of the locale. As solution could be to move the setlocale call from matplotlib's __init__ to __setitem__ of RcParams.
Setting rcParams['axes.formatter.use_locale'] to True in the config file also effects date formatting (%a, %A, %b, %B, %c,, %x, %X). This should be documented.
Maybe the locale should be reset to C when using any Gtk backend to be consitent with other backends, although this is a breaking API change. If we don't want to do this for backward compatibility reasons the specific behaviour of the Gtk backends with regard to locales should be documented.
Fix the wording of the Localization of date formats example (see comment above: [Bug]: Current locale not honored on Windows #25006 (comment))

Any thoughts, in particular to 3. and 1.?

Point 1. (using __setitem__) seems reasonable.
Regarding 3., our longstanding policy has been that if a GUI toolkit resets the locale, we should not undo that ourselves; see #6706 and the various linked issues.

@anntzer

Agree on 1 that moving this __setitem__ is a good idea.

Agree on 2 that it is worth pointing out the full extent of this setting.

On 3, I also agree with @anntzer that if the GUIs are doing things we should not get in the way at the backend level. A way my thinking has changed a bit from 2016 is that there might be a case for forcing the locale in pyplot. We (I) have started to think of pyplot as our "application" so doing work there to make the behavior consistent is probably reasonable (if you do not want us to "help", do not import pyplot!).

I'm also running into this bug. If I run the script from the comment above, I get the following

gtk3agg: raised ImportError
gtk3cairo: raised ImportError
gtk4agg: raised ImportError
gtk4cairo: raised ImportError
macosx: raised ImportError
nbagg: left C unchanged
notebook: left C unchanged
qtagg: changed locale from C to nl_NL.UTF-8
qtcairo: raised ImportError
qt5agg: raised ImportError
qt5cairo: raised ImportError
tkagg: left C unchanged
tkcairo: raised ImportError
webagg: left C unchanged
wx: raised ModuleNotFoundError
wxagg: raised ModuleNotFoundError
wxcairo: raised ModuleNotFoundError
agg: left C unchanged
cairo: raised ImportError
pdf: left C unchanged
pgf: left C unchanged
ps: left C unchanged
svg: left C unchanged
template: left C unchanged

qtagg is the default backend on my machine.

Which means in our scientific simulation library, a script can fail if it includes a matplotlib plotting call, since that changes the locale and afterwards we use tinyexpr (via Cython) to parse an expression. This program is locale dependent and only really works with the C numeric locale.

If anyone can give some pointers, I'd be happy to try and fix this, since it is a very annoying an subtle bug

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[Bug]: Current locale not honored on Windows #25006

Bug summary

Code for reproduction

Actual outcome

On Windows:

Expected outcome

On Linux

Additional information

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	if rcParams['axes.formatter.use_locale']:
	locale.setlocale(locale.LC_ALL, '')

[Bug]: Current locale not honored on Windows #25006

Description

Bug summary

Code for reproduction

Actual outcome

On Windows:

Expected outcome

On Linux

Additional information

Operating system

Matplotlib Version

Matplotlib Backend

Python version

Jupyter version

Installation

Activity

StefRe commented on Jan 16, 2023

StefRe commented on Jan 31, 2023

StefRe commented on Jan 31, 2023

anntzer commented on Jan 31, 2023

tacaswell commented on Jan 31, 2023

mironleon commented on Apr 9, 2026

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