import schema
import attr


_UNSET = object()


@attr.s
class AttrSchema(object):
    schema = attr.ib(convert=schema.Schema)
    default = attr.ib(_UNSET)

    def __call__(self, value):
        if value == self.default:
            return value
        return self.schema.validate(value)


def schema_attrib(scheme, default=_UNSET):
    if default is _UNSET:
        return attr.ib(convert=AttrSchema(scheme))
    return attr.ib(convert=AttrSchema(scheme, default), default=default)