Locale Bugfixes in Python 3

This article describes a few locales bugs that I fixed in Python 3 between 2012 (Python 3.3) and 2018 (Python 3.7):

  • Support non-ASCII decimal point and thousands separator
  • Crash with non-ASCII decimal point
  • LC_NUMERIC encoding different than LC_CTYPE encoding
  • LC_MONETARY encoding different than LC_CTYPE encoding
  • Tests non-ASCII locales

See also my previous locale bugfixes: Python 3, locales and encodings

Unicode Mixed Bag


Each language and each country has different ways to represent dates, monetary values, numbers, etc. Unix has "locales" to configure applications for a specific language and a specific country. For example, there are fr_BE for Belgium (french) and fr_FR for France (french).

In practice, each locale uses its own encoding and problems arise when an application uses a different encoding than the locale. There are LC_NUMERIC locale for numbers, LC_MONETARY locale for monetary and LC_CTYPE for the encoding. Not only it's possible to configure an application to use LC_NUMERIC with a different encoding than LC_CTYPE, but some users use such configuration!

In an application which only uses bytes for text, as Python 2 does mostly, it's mostly fine: in the worst case, users see mojibake, but the application doesn't "crash" (exit and/or data loss). On the other side, Python 3 is designed to use Unicode for text and fail with hard Unicode errors if it fails to decode bytes and fails to encode text.

Support non-ASCII decimal point and thousands separator

The Unicode type has been reimplemented in Python 3.3 to use "compact string": PEP 393 "Flexible String Representation". The new implementation is more complex and the format() function has been limited to ASCII for the decimal point and thousands separator (format a number using the "n" type).

In January 2012, Stefan Krah noticed the regression (compared to Python 3.2) and reported bpo-13706. I fixed the code to support non-ASCII in format (commit a4ac600d). But when I did more tests, I noticed that the "n" type doesn't decode properly the decimal point and thousands seprator which come from the localeconv() function which uses byte strings.

I fixed format(int, "n") with commit 41a863cb, decode decimal point and the thousands separator (localeconv() fields) from the locale encoding, rather than latin1, using PyUnicode_DecodeLocale():

commit 41a863cb81608c779d60b49e7be8a115816734fc
Author: Victor Stinner <victor.stinner@haypocalc.com>
Date:   Fri Feb 24 00:37:51 2012 +0100

    Issue #13706: Fix format(int, "n") for locale with non-ASCII thousands separator

     * Decode thousands separator and decimal point using PyUnicode_DecodeLocale()
       (from the locale encoding), instead of decoding them implicitly from latin1
     * Remove _PyUnicode_InsertThousandsGroupingLocale(), it was not used
     * Change _PyUnicode_InsertThousandsGrouping() API to return the maximum
       character if unicode is NULL
     * (...)

Note: I decided to not fix Python 3.2:

Hum, it is not trivial to redo the work on Python 3.2. I prefer to leave the code unchanged to not introduce a regression, and I wait until a Python 3.2 user complains (the bug exists since Python 3.0 and nobody complained).

Crash with non-ASCII decimal point

Six years later, in June 2018, I noticed that Python does crash when running tests on locales:

$ ./python
Python 3.8.0a0 (heads/master-dirty:bcd3a1a18d, Jun 23 2018, 10:31:03)
[GCC 8.1.1 20180502 (Red Hat 8.1.1-1)] on linux
>>> import locale
>>> locale.str(2.5)
>>> '{:n}'.format(2.5)

>>> locale.setlocale(locale.LC_ALL, '')
>>> locale.str(2.5)
>>> '{:n}'.format(2.5)
python: Objects/unicodeobject.c:474: _PyUnicode_CheckConsistency: Assertion `maxchar < 128' failed.
Aborted (core dumped)

I reported the issue as bpo-33954. The bug only occurrs for decimal point larger than U+00FF (code point greater than 255). It was a bug in my bpo-13706 fix: commit a4ac600d.

I pushed a second fix to properly support all cases, commit 59423e3d:

commit 59423e3ddd736387cef8f7632c71954c1859bed0
Author: Victor Stinner <vstinner@redhat.com>
Date:   Mon Nov 26 13:40:01 2018 +0100

    bpo-33954: Fix _PyUnicode_InsertThousandsGrouping() (GH-10623)

    Fix str.format(), float.__format__() and complex.__format__() methods
    for non-ASCII decimal point when using the "n" formatter.


    * Rewrite _PyUnicode_InsertThousandsGrouping(): it now requires
      a _PyUnicodeWriter object for the buffer and a Python str object
      for digits.
    * Rename FILL() macro to unicode_fill(), convert it to static inline function,
      add "assert(0 <= start);" and rework its code.

LC_NUMERIC encoding different than LC_CTYPE encoding

In August 2017, Petr Viktorin identified a bug in Koji (server building Fedora packages): UnicodeDecodeError in localeconv() makes test_float fail in Koji

"This is tripped by Python's test suite, namely test_float.GeneralFloatCases.test_float_with_comma"

He wrote a short reproducer script:

import locale
locale.setlocale(locale.LC_ALL, 'C.UTF-8')
locale.setlocale(locale.LC_NUMERIC, 'fr_FR.ISO8859-1')

Two months later, Charalampos Stratakis reported the bug upstream: bpo-31900. The problem arises when the LC_NUMERIC locale uses a different encoding than the LC_CTYPE encoding.

The bug was already known:

  • 2015-12-05: Serhiy Storchaka reported bpo-25812 with uk_UA locale
  • 2016-11-03: Guillaume Pasquet reported bpo-28604 with en_GB locale

Moreover, the bug was known since 2009, Stefan Krah reported a very similar bug: bpo-7442. I was even involved in this issue in 2013, but then I forgot about it (as usual, I am working on too many issues in parallel :-)).

In 2010, PostgreSQL had the same issue and fixed the bug by changing temporarily the LC_CTYPE locale to the LC_NUMERIC locale.

In January 2018, I came back to this 9 years old bug. I was fixing bugs in the implementation of my PEP 540 "Add a new UTF-8 Mode". I pushed a large change to fix locale encodings in bpo-29240, commit 7ed7aead:

commit 7ed7aead9503102d2ed316175f198104e0cd674c
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Mon Jan 15 10:45:49 2018 +0100

    bpo-29240: Fix locale encodings in UTF-8 Mode (#5170)

    Modify locale.localeconv(), time.tzname, os.strerror() and other
    functions to ignore the UTF-8 Mode: always use the current locale

    Changes: (...)

Stefan Krah asked:

I have the exact same questions as Marc-Andre. This is one of the reasons why I blocked the _decimal change. I don't fully understand the role of the new glibc, since #7442 has existed for ages -- and it is a open question whether it is a bug or not.

I replied:

Past 10 years, I repeated to every single user I met that "Python 3 is right, your system setup is wrong". But that's a waste of time. People continue to associate Python3 and Unicode to annoying bugs, because they don't understand how locales work.

Instead of having to repeat to each user that "hum, maybe your config is wrong", I prefer to support this non convential setup and work as expected ("it just works"). With my latest implementation, setlocale() is only done when LC_CTYPE and LC_NUMERIC are different, which is the corner case which "shouldn't occur in practice".

Marc-Andre Lemburg added:

Sounds like a good compromise :-)

After doing more tests on FreeBSD, Linux and macOS, I pushed commit cb064fc2 to fix bpo-31900 by changing temporarily the LC_CTYPE locale to the LC_NUMERIC locale:

commit cb064fc2321ce8673fe365e9ef60445a27657f54
Author: Victor Stinner <victor.stinner@gmail.com>
Date:   Mon Jan 15 15:58:02 2018 +0100

    bpo-31900: Fix localeconv() encoding for LC_NUMERIC (#4174)

    * Add _Py_GetLocaleconvNumeric() function: decode decimal_point and
      thousands_sep fields of localeconv() from the LC_NUMERIC encoding,
      rather than decoding from the LC_CTYPE encoding.
    * Modify locale.localeconv() and "n" formatter of str.format() (for
      int, float and complex to use _Py_GetLocaleconvNumeric()

I dislike my own fix because changing temporarily the LC_CTYPE locale impacts all threads, not only the current thread. But we failed to find another solution. The LC_CTYPE locale is only changed if the LC_NUMERIC locale is different than the LC_CTYPE locale and if the decimal point or the thousands separator is non-ASCII.

Note: I proposed a change to fix the same bug in the decimal module: PR #5191, but I abandonned my patch.

LC_MONETARY encoding different than LC_CTYPE encoding

Fixing bpo-31900 drained all my energy, but sadly... there was a similar bug with LC_MONETARY!

At 2016-11-03, Guillaume Pasquet reported bpo-28604: Exception raised by python3.5 when using en_GB locale.

The fix is similar to the LC_NUMERIC fix: change temporarily the LC_CTYPE locale to the LC_MONETARY locale, commit 02e6bf7f:

commit 02e6bf7f2025cddcbde6432f6b6396198ab313f4
Author: Victor Stinner <vstinner@redhat.com>
Date:   Tue Nov 20 16:20:16 2018 +0100

    bpo-28604: Fix localeconv() for different LC_MONETARY (GH-10606)

    locale.localeconv() now sets temporarily the LC_CTYPE locale to the
    LC_MONETARY locale if the two locales are different and monetary
    strings are non-ASCII. This temporary change affects other threads.


    * locale.localeconv() can now set LC_CTYPE to LC_MONETARY to decode
      monetary fields.
    * (...)

Tests non-ASCII locales

To test my bugfixes, I used manual tests. The first issue was to identify locales with problematic characters: non-ASCII decimal point or thousands separator for example. I wrote my own "test suite" for Windows, Linux, macOS and FreeBSD on my website: Test non-ASCII characters with locales.

Example with localeconv() on Fedora 27:

LC_ALL locale Encoding Field Bytes Text
es_MX.utf8 UTF-8 thousands_sep 0xE2 0x80 0x89 U+2009
fr_FR.UTF-8 UTF-8 currency_symbol 0xE2 0x82 0xAC U+20AC (€)
ps_AF.utf8 UTF-8 thousands_sep 0xD9 0xAC U+066C (٬)
uk_UA.koi8u KOI8-U currency_symbol 0xC7 0xD2 0xCE 0x2E U+0433 U+0440 U+043d U+002E (грн.)
uk_UA.koi8u KOI8-U thousands_sep 0x9A U+00A0

Manual tests became more and more complex, since there are so many cases: each operating system use different locale names and the result depends on the libc version. After months of manual tests, I wrote my small personal portable locale test suite: test_all_locales.py. It supports:

  • FreeBSD 11
  • macOS
  • Fedora (Linux)


def test_zh_TW_Big5(self):
    loc = "zh_TW.Big5" if BSD else "zh_TW.big5"
    if FREEBSD:
        currency_symbol = u'\uff2e\uff34\uff04'
        decimal_point = u'\uff0e'
        thousands_sep = u'\uff0c'
        date_str = u'\u661f\u671f\u56db 2\u6708'
        currency_symbol = u'NT$'
        decimal_point = u'.'
        thousands_sep = u','
        if MACOS:
            date_str =  u'\u9031\u56db 2\u6708'
            date_str = u'\u9031\u56db \u4e8c\u6708'

    self.set_locale(loc, "Big5")

    lc = locale.localeconv()
    self.assertLocaleEqual(lc['currency_symbol'], currency_symbol)
    self.assertLocaleEqual(lc['decimal_point'], decimal_point)
    self.assertLocaleEqual(lc['thousands_sep'], thousands_sep)

    self.assertLocaleEqual(time.strftime('%A %B', FEBRUARY), date_str)

The best would be to integrate directly these tests into the Python test suite, but it's not portable nor future-proof, since most constants are hardcoded and depends on the operating sytem and the libc version.