python - Why is there no line break in the result after writing a newline statement?

Question

I want to write the numbers after the colon of the file to another file in each line, and wrap the line at the end of each line. Use the following code, there is a line break statement, but the result is still no line break, what should I do?

#!/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')

Answer 1

The line break on windows should be rn. So file.write('rn').

Answer 2

The indentation is wrong.

Answer 3

with open('a.txt', 'r') as f1, open('b.txt', 'a') as f2:
    for line in f1:
        new_line = [_.split(':')[1] for _ in line.split()]
        f2.write(' '.join(new_line))
        f2.write('\n')