docs: def get_differentially_expressed_genes(self, diff_type: str) -> VertexSeq:
          """Get the differentially expressed genes based on diff_type.
  
          :param str diff_type: Differential expression type chosen by the user; all, down, or up.
          :return list: A list of differentially expressed genes.
          """
          if diff_type == "up":
              diff_expr = self.graph.vs.select(up_regulated_eq=True)
          elif diff_type == "down":
              diff_expr = self.graph.vs.select(down_regulated_eq=True)
          else:
              diff_expr = self.graph.vs.select(diff_expressed_eq=True)
          return diff_expr