format(value) and value.__format__() behave differently even though the documentation says otherwise:

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

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

The difference happens when the format string is unicode.  For example:

>>> format(10, u'n')
u'10'
>>> (10).__format__(u'n')  # parentheses needed to prevent SyntaxError
'10'

So either the documentation should be changed, or the behavior should be changed to match.

Related to this: neither the "Format Specification Mini-Language" documentation nor the string.Formatter docs seem to say anything about the effect that a unicode format string should have on the return value (in particular, should it cause the return value to be unicode or not):

http://docs.python.org/library/string.html#formatspec
http://docs.python.org/library/string.html#string-formatting

See also issue 15276 (int formatting), issue 15951 (empty format string), and issue 7300 (unicode arguments).