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msg95988> Hi, the following works in 2.7 but not in 3.x: ...
Sure it works because Python 2 pass the raw byte string, it does not try to decode it. But did you try to display the result in a terminal for example?
Example with Python 2 in an UTF-8 terminal:
$ python
Python 2.7.5 (default, Oct 8 2013, 12:19:40) 
[GCC 4.8.1 20130603 (Red Hat 4.8.1-1)] on linux2
>>> import locale
>>> # set the locale encoding to UTF-8
... locale.setlocale(locale.LC_CTYPE, 'fr_FR.utf8')
'fr_FR.utf8'
>>> # set the thousand separator to U+00A0
... locale.setlocale(locale.LC_NUMERIC, 'fi_FI')
'fi_FI'
>>> locale.getlocale(locale.LC_CTYPE)
('fr_FR', 'UTF-8')
>>> locale.getlocale(locale.LC_NUMERIC)
('fi_FI', 'ISO8859-15')
>>> locale.format('%d', 123456, True)
'123\xa0456'
>>> print(locale.format('%d', 123456, True))
123�456
Mojibake! � means that b'\xA0' cannot be decoded from the locale encoding (UTF-8).
There is probably the same issue with a LC_MONETARY using a different encoding than LC_CTYPE.
> I suspect that this is related: #5905
It is unrelated: time.strftime() uses the LC_CTYPE, but the Python was using the wrong encoding. Python used the locale encoding read at startup, whereas the *current* locale encoding must be used.
This issue is specific to LC_NUMERIC with a LC_CTYPE using different encoding.
> If I set LC_CTYPE and LC_NUMERIC together, things work.
Sure, because in this case, LC_NUMERIC produces data in the same encoding than LC_CTYPE.
> call setlocale(LC_CTYPE, setlocale(LC_NUMERIC, NULL)) before
> mbstowcs. This is not really an option.
Setting a locale is process-wide and should be avoided. FYI locale.getpreferredencoding() changes temporarly the LC_CTYPE by default, it only uses the current LC_CTYPE if you pass False. open() changed temporarly LC_CTYPE because of that in Python 3.0-3.2 (see issue #11022).
The following PostgreSQL issue looks to be the same than this Python issue:
4B7E07541D0@cvs.postgresql.org">http://www.postgresql.org/message-id/20100422015552.4B7E07541D0@cvs.postgresql.org
The fix changes temporarly the LC_CTYPE encoding:
#ifdef WIN32
setlocale(LC_CTYPE, locale_monetary);
#endif
(I don't know why the code is specific to Windows.)