'Fails if the given ``item`` is not a byte string.
Use `Should Be Unicode String` if you want to verify the ``item`` is a
Unicode string, or `Should Be String` if both Unicode and byte strings
are fine. 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_be_byte_string(self, item, msg=None):

    if (not is_bytes(item)):
        self._fail(msg, "'%s'    is    not    a    byte    string.", item)
