I can't yet reproduce on my system, but after looking at the code, I believe the following are closer to the cause:

>>> format(10000, u'n')
>>> int.__format__(10000, u'n')

Incidentally, on my system, the following note in the docs is wrong:

"Note: format(value, format_spec) merely calls value.__format__(format_spec)."

(from http://docs.python.org/library/functions.html?#format )

>>> format(10000, u'n')
u'10000'
>>> 10000.__format__(u'n')
  File "<stdin>", line 1
    10000.__format__(u'n')
                   ^
SyntaxError: invalid syntax
>>> int.__format__(10000, u'n')
'10000'

Observe also that format() and int.__format__() return different types.