python 字典嵌套

import csv
data = csv.reader(open("data_data.csv","r"))
datatrain = open("train.csv","w")
datatest = open("test.csv","w")
cell = {}

for line in data:
    if line[2] not in cell:
        cell[line[2]] ={}
        cell[line[2]][line[3]]=[",".join(line[4:])]
    else:
        if line[3] not in cell[line[2]]:
            cell[line[2]][line[3]] = [",".join(line[4:])]
        else:
            cell[line[2]][line[3]].append(",".join(line[4:]))