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I was surprised falling back to ascii actually worked.  I
found a bunch of docs at the devoper.apple.com site.  As far
as I can tell, the proper way to fix this is to add the
necessary support to macglue.h, etc. so the functions in
_locale work on OS X as well.

I found out why this only just now bit me: I had compatible
libs in /usr/local/lib which made nl_langinfo() available in
_locale until I upgraded to 10.2.1.

The proper fix is going to require some effort by someone
who understands Apple API's much better than I do.

Since falling back to ascii works (at least for en_US) as
you intended - you were the one who added the locale code to
IOBinding.py - perhaps a temporary fix is to make sure the
fallback works as you intended?