code: def _getGraph(self, graph_name, fail_noexist=False):
          
          graph = self._graphDict.get(graph_name)
          if fail_noexist and graph is None:
              raise AttributeError("Invalid graph name: %s" % graph_name)
          else:
              return graph
docs: Private method for returning graph object with name graph_name. 
          
          @param graph_name:   Graph Name
          @param fail_noexist: If true throw exception if there is no graph with
                               name graph_name.
          @return:             Graph Object or None