code: def betweenness_centrality(self, normalized=True):
          
          bc = proximity.brandes_betweenness_centrality(self, normalized)
          for id, w in bc.iteritems(): self[id]._betweenness = w
          return bc
docs: Calculates betweenness centrality and returns an node id -> weight dictionary.
          Node betweenness weights are updated in the process.