SimpleGraph Python In-Memory TripleStore

import csv

class SimpleGraph:
    def __init__(self):
        self._spo = {}
        self._pos = {}
        self._osp = {}

    def add(self, (sub, pred, obj)):
        """
        Adds a triple to the graph.
        """
        self._addToIndex(self._spo, sub, pred, obj)
        self._addToIndex(self._pos, pred, obj, sub)
        self._addToIndex(self._osp, obj, sub, pred)

    def _addToIndex(self, index, a, b, c):
        """
        Adds a triple to a specified index.
        """
        if a not in index: index[a] = {b:set([c])}
        else:
            if b not in index[a]: index[a][b] = set([c])
            else: index[a][b].add(c)

    def remove(self, (sub, pred, obj)):
        """
        Remove a triple pattern from the graph.
        """
        triples = list(self.triples((sub, pred, obj)))
        for (delSub, delPred, delObj) in triples:
            self._removeFromIndex(self._spo, delSub, delPred, delObj)
            self._removeFromIndex(self._pos, delPred, delObj, delSub)
            self._removeFromIndex(self._osp, delObj, delSub, delPred)

    def _removeFromIndex(self, index, a, b, c):
        """
        Removes a triple from an index and clears up empty indermediate structures.
        """
        try:
            bs = index[a]
            cset = bs[b]
            cset.remove(c)
            if len(cset) == 0: del bs[b]
            if len(bs) == 0: del index[a]
        # KeyErrors occur if a term was missing, which means that it wasn't a valid delete:
        except KeyError:
            pass

    def triples(self, (sub, pred, obj)):
        """
        Generator over the triple store.
        Returns triples that match the given triple pattern.
        """
        # check which terms are present in order to use the correct index:
        try:
            if sub != None:
                if pred != None:
                    # sub pred obj
                    if obj != None:
                        if obj in self._spo[sub][pred]: yield (sub, pred, obj)
                    # sub pred None
                    else:
                        for retObj in self._spo[sub][pred]: yield (sub, pred, retObj)
                else:
                    # sub None obj
                    if obj != None:
                        for retPred in self._osp[obj][sub]: yield (sub, retPred, obj)
                    # sub None None
                    else:
                        for retPred, objSet in self._spo[sub].items():
                            for retObj in objSet:
                                yield (sub, retPred, retObj)
            else:
                if pred != None:
                    # None pred obj
                    if obj != None:
                        for retSub in self._pos[pred][obj]:
                            yield (retSub, pred, obj)
                    # None pred None
                    else:
                        for retObj, subSet in self._pos[pred].items():
                            for retSub in subSet:
                                yield (retSub, pred, retObj)
                else:
                    # None None obj
                    if obj != None:
                        for retSub, predSet in self._osp[obj].items():
                            for retPred in predSet:
                                yield (retSub, retPred, obj)
                    # None None None
                    else:
                        for retSub, predSet in self._spo.items():
                            for retPred, objSet in predSet.items():
                                for retObj in objSet:
                                    yield (retSub, retPred, retObj)
        # KeyErrors occur if a query term wasn't in the index, so we yield nothing:
        except KeyError:
            pass

    def value(self, sub=None, pred=None, obj=None):
        for retSub, retPred, retObj in self.triples((sub, pred, obj)):
            if sub is None: return retSub
            if pred is None: return retPred
            if obj is None: return retObj
            break
        return None

    def load(self, filename):
        f = open(filename, "rb")
        reader = csv.reader(f)
        for sub, pred, obj in reader:
            sub = unicode(sub, "UTF-8")
            pred = unicode(pred, "UTF-8")
            obj = unicode(obj, "UTF-8")
            self.add((sub, pred, obj))
        f.close()

    def save(self, filename):
        f = open(filename, "wb")
        writer = csv.writer(f)
        for sub, pred, obj in self.triples((None, None, None)):
            writer.writerow([sub.encode("UTF-8"), pred.encode("UTF-8"), obj.encode("UTF-8")])
        f.close()

if __name__ == "__main__":
    g = SimpleGraph()
    g.add(("blade_runner", "name", "Blade Runner"))
    g.add(("blade_runner", "name", "Blade Runner"))
    g.add(("blade_runner", "release_date", "June 25, 1982"))
    g.add(("blade_runner", "directed_by", "Ridley Scott"))

    print list(g.triples((None, None, None)))
    print list(g.triples(("blade_runner", None, None)))
    print list(g.triples(("blade_runner", "name", None)))
    print list(g.triples(("blade_runner", "name", "Blade Runner")))
    print list(g.triples(("blade_runner", None, "Blade Runner")))
    print list(g.triples((None, "name", "Blade Runner")))
    print list(g.triples((None, None, "Blade Runner")))

    print list(g.triples(("foo", "name", "Blade Runner")))
    print list(g.triples(("blade_runner", "foo", "Blade Runner")))
    print list(g.triples(("blade_runner", "name", "foo")))