Okay, so line 187 of locale.py has this test:

    if not match or len(match.group())!= len(percent):

the problematic part is the len test.  When format string is '%.0f KB' match.group() is '%.0f' but of course percent is the full string.

This seems like a bogus test, since clearly the given input is a valid format string.  I'm not sure what the intent of this test is.  The Python 2.6 test is:

    if percent[0] != '%':

which is perhaps too naive.

I guess I don't understand why this test is here.  Wouldn't it make more sense to either just let any TypeError from _format() to percolate up, or to catch that TypeError and transform it into the ValueError?  Why try to replicate the logic of str.__mod__()?