def adapt_gene_id(gene_id, longest_gene_number_length):
    """
    Input: a gene ID like g_1 and the longest_gene_number_length (5 if you have a gene like g_12356).
    Return a new gene ID with a completion of 0 like: g_00001.
    """
    gene_prefix = '_'.join(gene_id.split('_')[:-1])
    gene_number = gene_id.split('_')[-1]
    new_gene_id = gene_prefix + '_' + gene_number.zfill(
        longest_gene_number_length)
    return new_gene_id
