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It used to be possible to specify Unicode fill characters in numeric
formatting:
Python 3.3.0a0 (default:1dd6908df8f5, Jul 16 2011, 11:16:00) 
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> format(1234, "\u2007<7")
'1234\u2007\u2007\u2007'
[64682 refs]
>>> 
Now it doesn't work:
Python 3.3.0a0 (default:65ac469d30e6, Jan 3 2012, 23:23:07) 
[GCC 4.4.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> format(1234, "\u2007<7")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: fill character too large

It's still possible; it's just apparently limited to ASCII characters.

Hum, somehow I always refuse to acknowledge that ASCII is a subset
of Unicode. :)

I assume this is left over from the PEP 393 changes. I think the right thing to do is delete this code from line 277 of formatter_unicode.c:
 if (format->fill_char > 127 || format->align > 127 ||
 format->sign > 127) {
 PyErr_SetString(PyExc_ValueError, "fill character too large");
 return 0;
 }
I'm not sure such a restriction needs to exist any more. But I'll admit to not having thought it through.

> I assume this is left over from the PEP 393 changes.
Correct.
> I'm not sure such a restriction needs to exist any more.
The restriction was introduced to simplify the implementation. maxchar has to be computed exactly in format_string_internal(), format_int_or_long_internal(), format_float_internal() and format_complex_internal().
For format_string_internal(), the following change is enough (untested):
 if (lpad != 0 || rpad != 0)
 maxchar = Py_MAX(maxchar, format->fill_char);
For number formatting functions, spec->n_lpadding, spec->n_spadding and spec->n_rpadding have to be checked. Something like:
 if (spec->n_lpadding || spec->n_spadding || spec->n_rpadding)
 maxchar = Py_MAX(maxchar, format->fill_char);

Removing the if condition would be incorrect. The maximum char is computed at the beginning of the formatting. If, during formatting, need for a padding character is determined, the padding character must not be larger than the maximum char of the target string - which is 127 unless 'c' formatting is used.
One solution would be to determine whether the padding character is used in advance. Another solution would be to widen the string when the need for non-ASCII padding is detected.
I have no intention of fixing this issue myself; I don't mind if non-ASCII padding characters are not supported in 3.3.

Actually the issue is not restricted to numeric formatting. It's not
possible to pad a Unicode string with a non-ascii whitespace:
>>> format("abcd", "\u2007<7")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: fill character too large
I'd be more than happy to restrict all numerical I/O operations to
ASCII. This includes input strings for int(), float(), Decimal().
It does break backwards compatibility though and the situation
for string formatting above seems odd to me. It is worse when
the rejected fill character is already present in the string:
>>> format("\u2007abcd", "\u2007<7")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: fill character too large

Sorry for the off-the-cuff diagnosis. I had assumed this was the unintended result of the conversion, but of course I'm wrong.
I'd like to fix this.

New changeset 231c6042c40c by Victor Stinner in branch 'default':
Issue #13706: Support non-ASCII fill characters
http://hg.python.org/cpython/rev/231c6042c40c 

I fixed the original report, but there is still an issue with non-ASCII thousands separator.

New changeset 056f5cc8885d by Victor Stinner in branch 'default':
Issue #13706: Add assertions to detect bugs earlier
http://hg.python.org/cpython/rev/056f5cc8885d 

With the changeset 056f5cc8885d, format(1234, 'n') fails in a locale using a non-ASCII thousands separator. Patch showing the problem:
diff --git a/Lib/test/test_format.py b/Lib/test/test_format.py
--- a/Lib/test/test_format.py
+++ b/Lib/test/test_format.py
@@ -1,4 +1,5 @@
 from test.support import verbose, TestFailed
+import locale
 import sys
 import test.support as support
 import unittest
@@ -282,6 +283,19 @@ class FormatTest(unittest.TestCase):
 self.assertEqual(format(1+2j, "\u2007^8"), "\u2007(1+2j)\u2007")
 self.assertEqual(format(0j, "\u2007^4"), "\u20070j\u2007")
 
+ def test_locale(self):
+ try:
+ oldloc = locale.setlocale(locale.LC_ALL, '')
+ except locale.Error as err:
+ self.skipTest("Cannot set locale: {}".format(err))
+ try:
+ sep = locale.localeconv()['thousands_sep']
+ self.assertEqual(format(123456789, "n"),
+ sep.join(('123', '456', '789')))
+ finally:
+ locale.setlocale(locale.LC_ALL, oldloc)
+
+
 
 def test_main():
 support.run_unittest(FormatTest)

thousands_separator.patch:
 - Remove the unused _PyUnicode_InsertThousandsGroupingLocale() function
 - Add a test for non-ASCII thousands separator (depend on the locale)
 - _PyUnicode_InsertThousandsGrouping() expects thousands separator as a Unicode object and return the maximum character if unicode is NULL
 - Fix str.__format__() for non-ASCII thousands separator
Note: the patch handles also non-ASCII decimal point, even I don't know any locale using such point.

I'm using ps_AF for testing:
$ ./localeconv_wchar ps_AF
size of wchar_t: 32 bits
decimal_point byte string: "\xd9\xab" (2 bytes)
decimal_point wide string: L"\u066b" (1 characters)
thousands_sep byte string: "\xd9\xac" (2 bytes)
thousands_sep wide string: L"\u066c" (1 characters)
currency_symbol byte string: "\xd8\xa7\xd9\x81\xd8\xba\xd8\xa7\xd9\x86\xdb\x8d" (12 bytes)
currency_symbol wide string: L"\u0627\u0641\u063a\u0627\u0646\u06cd" (6 characters)

New changeset f89e2f4cda88 by Victor Stinner in branch 'default':
Issue #13706: Fix format(int, "n") for locale with non-ASCII thousands separator
http://hg.python.org/cpython/rev/f89e2f4cda88 

The ps_AF locale fails with an assert after the latest commit:
Python 3.3.0a0 (default:f89e2f4cda88+, Feb 24 2012, 01:14:50) 
[GCC 4.4.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import locale
[67103 refs]
>>> locale.setlocale(locale.LC_ALL, "ps_AF")
'ps_AF'
[67108 refs]
>>> format(13232434234.23423, "n")
python: Python/formatter_unicode.c:606: fill_number: Assertion `r == spec->n_grouped_digits' failed.
Aborted

New changeset a29d20fa85b4 by Victor Stinner in branch 'default':
Issue #13706: Fix format(float, "n") for locale with non-ASCII decimal point (e.g. ps_aF)
http://hg.python.org/cpython/rev/a29d20fa85b4 

> The ps_AF locale fails with an assert after the latest commit:
> ...
> format(13232434234.23423, "n")
> python: Python/formatter_unicode.c:606: fill_number: 
> Assertion `r == spec->n_grouped_digits' failed.
> Aborted
Oh, you found a locale with a non-ASCII decimal point, cool! I failed to find such locale. The last commit makes Python supports non-ASCII decimal point.
Your comment is incorrect, it was already failing before my commit ;-) Example at changeset 548a023c8230:
$ LANG=ps_AF ./python 
Python 3.3.0a0 (default:548a023c8230, Feb 24 2012, 01:48:01) 
>>> import locale
>>> locale.setlocale(locale.LC_ALL, 'ps_AF')
'ps_AF'
>>> format(0.1, 'n')
python: Objects/unicodeobject.c:391: _PyUnicode_CheckConsistency: Assertion `maxchar < 128' failed.
Abandon
--
By the way, Python 3.2 fails also to handle non-ASCII thousands separator or non-ASCII decimal point:
$ LANG=ps_AF python3
Python 3.2.1 (default, Jul 11 2011, 18:54:42) 
[GCC 4.6.1 20110627 (Red Hat 4.6.1-1)] on linux2
>>> import locale
>>> locale.setlocale(locale.LC_ALL, 'ps_AF')
'ps_AF'
>>> format(1234, 'n')
'1\Uffffffd9\Uffffffac234'
>>> format(0.1, 'n')
'0\Uffffffd9\Uffffffab1'
D9 AC/AB are byte strings b'\xD9\xAC' and b'\xD9\xAB' which are UTF-8 encode strings corresponding to U+066C (arabic thousands separator) and U+066B (arabic decimal separator).
\Uffffffab is a bug in a cast from signed char to 32-bit unsigned integer (Py_UNICODE on Linux).

STINNER Victor <report@bugs.python.org> wrote:
> Your comment is incorrect, it was already failing before my commit ;-) Example at changeset 548a023c8230:
Ah, sorry about that. I was lazy and tested against 585d3664da89 (which is a
couple of weeks old).
That version didn't crash but produced UTF-8 output. :)
>>> format(0.1, 'n')
'0\xd9\xab1'
Now things look great, thanks for fixing all of that.

> By the way, Python 3.2 fails also to handle
> non-ASCII thousands separator or non-ASCII
> decimal point: (...)
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).