在python中使用with打开多个文件的方法

with open('file1') as f1:
 with open('file2') as f2:
  with open('file3') as f3:
   for i in f1:
    j = f2.readline()
    k = f3.readline()
    print(i,j,k)

for i,j,k in f1,f2,f3:
 print(i,j,k)

with open('file1') as f1, open('file2') as f2, open('file3') as f3:
 for i in f1:
  j = f2.readline()
  k = f3.readline()
  print(i,j,k)

from contextlib import nested #这个包是python2中的，不使用python3
with nested(open('file1'), open('file2'), open('file3')) as (f1,f2,f3):
 for i in f1:
  j = f2.readline()
  k = f3.readline()
  print(i,j,k)