--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rtags.py	Thu Mar 26 18:59:56 2009 +0100
@@ -0,0 +1,68 @@
+
+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__