用python把一个txt文件中所有逗号，替换成空格？

 string = "word  2     3  4    5 6     7"
 string = ",".join(string.split())

 import numpy as np
 import pandas as pd
 a = pd.read_csv('list_landmarks_align_celeba.txt')
 print(a)
 f = open("list_landmarks_align_celeba.txt","r")
 for line in f.readlines():
     line = ",".join(line.split())
     print(line)
     print(line[3])
     # print(line[ :2])
     # print(line[::2])