class FCollection():
    def __init__(self):
        self.items = [] #data
        self.tblIdxLookup = {} #key->index
        self.tblKeyLookup = [] #index->key
        self.tblFreeKeys = [] #freed keys

    def add(self, v):
        if len(self.tblFreeKeys):
            k = self.tblFreeKeys.pop()
        else: k = len(self.tblKeyLookup)
        self.items.append(v)
        self.tblIdxLookup[k] = len(self.tblKeyLookup)
        self.tblKeyLookup.append(k)

    def get(self, idx):
        return self.items[idx]

    def getByKey(self, key):
        return self.items[self.tblIdxLookup[key]]

    def remove(self, idx):
        key = self.tblKeyLookup.pop(idx)
        del self.tblIdxLookup[key]
        self.tblFreeKeys.append(key)
        del self.items[idx]

    def removeByKey(self, key):
        idx = self.tblIdxLookup.pop(key)
        del self.tblKeyLookup[idx]
        self.tblFreeKeys.append(key)
        del self.items[idx]

    def key(self, idx):
        return self.tblKeyLookup[idx]

    def index(self, key):
        return self.tblIdxLookup[key]

fc = FCollection()

fc.add("along")
fc.add("comes")
fc.add("Mary")
key = fc.key(1)
fc.removeByKey(key)
print fc.get(1)