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.

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