Message8848

This is now fixed in http://hg.python.org/jython/rev/6cffc2f6a643 (I claim). unicode._formatter_field_name_split is needed as well.

Jython 2.7b3+ (default:5efdcedc9817+6cffc2f6a643+, Jun 29 2014, 15:46:14)
[Java HotSpot(TM) 64-Bit Server VM (Oracle Corporation)] on java1.7.0_60
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>>> list(u"hello {:{0}d} and {}"._formatter_parser())
[(u'hello ', u'0', u'{0}d', None), (u' and ', u'1', u'', None)]
>>> first, rest = u"a.b[2]"._formatter_field_name_split()
>>> first, list(rest)
(u'a', [(True, u'b'), (False, 2)])
>>> import string
>>> string.Formatter().format(u"{} {:*>12d} {:{width}.{prec}f}", 10, 20, 30, width=8, prec=3)
u'10 **********20   30.000'
>>> string.Formatter().format(u"{0}", 10)
'10'
>>>

The last one is interesting because the problem is in string.py (it produces a str). It happens only if the format contains no literal text at all (just replacement fields) and no unicode arguments:
>>> string.Formatter().format(u"{a}", a=u"b")
u'b'
>>> string.Formatter().format(u"{a}", a="b")
'b'
This appears to be a "won't fix" for CPython. See http://bugs.python.org/issue15951 .