Add the object to the cache.

def Put(self, key, obj):
    """Add the object to the cache."""
    # Remove the old entry if it is there.
    node = self._hash.pop(key, None)
    if node:
      self._age.Unlink(node)

    # Make a new node and insert it.
    node = Node(key=key, data=obj)
    self._hash[key] = node
    self._age.AppendNode(node)

    self.Expire()

    return key