'Signed-ness for packing operation when it\'s not explicitly set.
Can be set to any non-string truthy value, or the specific string
values ``\'signed\'`` or ``\'unsigned\'`` which are converted into
:const:`True` and :const:`False` correspondingly.
Examples:
>>> context.signed
False
>>> context.signed = 1
>>> context.signed
True
>>> context.signed = \'signed\'
>>> context.signed
True
>>> context.signed = \'unsigned\'
>>> context.signed
False
>>> context.signed = \'foobar\' #doctest: +ELLIPSIS
Traceback (most recent call last):
AttributeError: signed must be one of [\'no\', \'signed\', \'unsigned\', \'yes\'] or a non-string truthy value'

@_validator
def signed(self, signed):

    try:
        signed = ContextType.signednesses[signed]
    except KeyError:
        pass
    if isinstance(signed, str):
        raise AttributeError(('signed    must    be    one    of    %r    or    a    non-string    truthy    value' % sorted(ContextType.signednesses)))
    return bool(signed)
