code: def get_by_index(self, i):
          
          if i >= self.n:
              raise ValueError('Index %d out of bounds ' % i +
                               'for database with %d gene sets.' % self.n)
          return self._gene_sets[self._gene_set_ids[i]]
docs: Look up a gene set by its index.
  
          Parameters
          ----------
          i: int
              The index of the gene set.
  
          Returns
          -------
          GeneSet
              The gene set.
  
          Raises
          ------
          ValueError
              If the given index is out of bounds.