pyhon-在原文本文件中替换某些特定的字符

# coding:utf-8
# 20170718_N.csv 文件的格式为：
# "1",2,3,"4","5",6,"7",8,9,10,11,12,13,14,15,"16",17,18,19,20,21,22,23,24，25
# 要求将前23个","号替换为"|",且在原文件中进行修改
op = open('20170718_N.csv', 'r+')
line_tell = 0
for line in op:
count = 0
for i in line:
line_tell += 1
op.seek(line_tell, 0)
if i == ',':
if count >= 23:
break
            else:
op.seek(line_tell - 1, 0)
op.write("|")
count += 1
else:
continue
op.close()