python 两个文件内容重叠部分

f1 = open('a.txt', 'r')
s1 = f1.read()

f2 = open('b.txt', 'r')
s2 = f2.read()

ls = list(set(s1.split('\n')) & set(s2.split('\n')))

f3 = open('重叠部分.txt', 'w')

for i in ls:
    f3.write(i)
    f3.write('\n')

f1.close()
f2.close()
f3.close()