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Which causes the locale machinery to spit exceptions, and the program to 
die, usually (eg, hg).
This manifests naturally on an Intel Mac, Mac OS X 10.5.7, but the 
problem behavior is in _build_localename. When called as
_build_localename((None,'any_string'))
it returns 'C.any_string'. I don't know of any system that supports 
anything but the POSIX portable character set in the C/POSIX locale, so
this is clearly wrong.
I suggest that when the first component of the argument is None, the
second component should be ignored.
Probably my Mac is misconfigured, but I think this is still a bug that 
should be fixed.
Observed in all of 2.5.4, 2.6.2, and 3.0.1 (vanilla MacPorts builds).
References: It's possible this is related to issue1699853, issue1176504, 
issue504219, but I don't think fixing this will help with those issues. 
It is not related to issue3067.

This was probably fixed by the checkins for Issue6202.
$ python3.1
Python 3.1rc1+ (py3k, Jun 8 2009, 22:53:59) 
[GCC 4.0.1 (Apple Inc. build 5490)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import locale
>>> locale._build_localename(locale.getdefaultlocale())
'en_US.UTF8'
>>> 
$ python3.0
Python 3.0.1 (r301:69597, Feb 14 2009, 19:03:52) 
[GCC 4.0.1 (Apple Inc. build 5490)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import locale
>>> locale._build_localename(locale.getdefaultlocale())
'C.mac-roman'
>>>

Also seeing this was resolved by Issue6202.
Python 3.1rc2+ (py3k:73552, Jun 24 2009, 23:11:23) 
[GCC 4.0.1 (Apple Inc. build 5490)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import locale
>>> locale._build_localename(locale.getdefaultlocale())
'en_US.UTF8'
Python 3.0.1+ (release30-maint:73553, Jun 25 2009, 08:35:35) 
[GCC 4.0.1 (Apple Inc. build 5490)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import locale
>>> locale._build_localename(locale.getdefaultlocale())
'C.mac-roman'