想把文件冒号后面的数字，按照每行写入另一个文件，在每行末尾换行，用以下代码，有换行语句，结果还是没换行，怎么办？

#!/usr/bin/python
#coding:utf-8
import pickle
import re
tfidf_dict={}
tfidf_all=[]
with open('/home/user1/zhouchun/lda/KNN/single_tfidf.txt','a') as file:
with open('/home/user1/zhouchun/lda/KNN/train_tfidf.txt', 'r') as fw:
        # content = fw.readlines()
        for line in fw:
            index_tfidf=line.split()
            # print index_tfidf
            for j in index_tfidf:
                m=re.compile(r'^(\d+):(\d+)$')
                try:
                    word_index=m.match(j).group(1)
                    tfidf=m.match(j).group(2)
                    file.write(str(tfidf) + ' ')
                except:
                     word_index=None
                     tfidf=None
        file.write('\n')