rep = ''
with open('1.txt', 'r') as fp:
contents = fp.read()
//read()方法读取所有内容，得到字符串。readline()读取一行内容，得到字符串。readlines()读取所有内容，按行得到字符串列表
rep = contents.replace(' ', ',')
with open('1.txt', 'w') as fp2:
//用'w'方式打开文件用于写入，写入时原文件的内容会被清空。 python没有writeline()、writelines()方法
fp2.write(rep)