class RelationTags(object):

    def __init__(self, use_set=False):

        self.use_set = use_set

        self._tagdefs = {}

    def set_rtag(self, tag, role, rtype, stype='*', otype='*'):

        assert not self.use_set

        assert role in ('subject', 'object'), role

        self._tagdefs[(stype, rtype, otype, role)] = tag

    def rtag(self, role, rtype, stype='*', otype='*'):

        assert not self.use_set

        for key in reversed(self._get_keys(role, rtype, stype, otype)):

            try:

                return self._tagdefs[key]

            except KeyError:

                continue

        return None

    def etype_rtag(self, etype, role, rtype, ttype='*'):

        if role == 'subject':

            return self.rtag(role, rtype, etype, ttype)

        return self.rtag(role, rtype, ttype, etype)

    def add_rtag(self, tag, role, rtype, stype='*', otype='*'):

        assert self.use_set

        assert role in ('subject', 'object'), role

        rtags = self._tagdefs.setdefault((stype, rtype, otype, role), set())

        rtags.add(tag)

    def rtags(self, role, rtype, stype='*', otype='*'):

        assert self.use_set

        rtags = set()

        for key in self._get_keys(role, rtype, stype, otype):

            try:

                rtags.update(self._tagdefs[key])

            except KeyError:

                continue

        return rtags

    def etype_rtags(self, etype, role, rtype, ttype='*'):

        if role == 'subject':

            return self.rtags(role, rtype, etype, ttype)

        return self.rtags(role, rtype, ttype, etype)

    def _get_keys(self, role, rtype, stype, otype): 

        assert role in ('subject', 'object'), role

        keys = [('*', rtype, '*', role),

                ('*', rtype, otype, role),

                (stype, rtype, '*', role),

                (stype, rtype, otype, role)]

        if stype == '*' or otype == '*':

            keys.remove(('*', rtype, '*', role))

            if stype == '*':

                keys.remove(('*', rtype, otype, role))

            if otype == '*':

                keys.remove((stype, rtype, '*', role))            

        return keys

    # dict compat

    def __getitem__(self, key):

        if isinstance(key, basestring):

            key = (key,)

        return self.rtags(*key)

    __contains__ = __getitem__