python with 打开多个文件

考虑如下的案例： 
同时打开三个文件，文件行数一样，要求实现每个文件依次读取一行，然后输出，我们先来看比较容易想到的写法：

with open(filename1,'rb') as fp1:
    with open(filename2,'rb') as fp2:
        with open(filename3,'rb') as fp3:
            for i in fp1:
                j = fp2.readline()
                k = fp3.readline()
                print(i,j,k)

注意这里只能对单个文件进行for循环读取，不能写成：

for i,k in fp1,fp2,fp3:
    print(i,k)

但可使用强大的zip操作：

for i,k in zip(fp1,fp3):
    print(i,k)

这样层层的嵌套未免啰嗦，with结构支持一种更简洁的写法：

with open(filename1,'rb') as fp1,open(filename2,'rb') as fp2,open(filename3,'rb') as fp3:
    for i in fp1:
        j = fp2.readline()
        k = fp3.readline()
        print(i,k)

或者使用更为优雅的写法，此时需要contextlib语法糖：

from contextlib improt ExitStack
with ExitStack() as stack:
    files = [stack.enter_context(open(fname)) for fname in [filename1,filename2,filename3]]
    for i,k in zip(files[0],files[1],files[2]):
        print(i,k)