'Fails if the given ``item`` is a string.
See `Should Be String` for more details about Unicode strings and byte
strings.
The default error message can be overridden with the optional
``msg`` argument.'

def should_not_be_string(self, item, msg=None):

    if is_string(item):
        self._fail(msg, "'%s'    is    a    string.", item)
