Python使用with结构打开多个文件

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 f1:
    with open(filename2, 'rb') as f2:
        with open(filename3, 'rb') as f3:
            for i in f1:
                j = f2.readline()
                k = f3.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, j, k in f1, f2, f3:
    print(i, j, k)

但可使用强大的zip操作：

for i, j, k in zip(f1, f2, f3):
    print(i, j, 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 f1, open(filename2, 'rb') as f2, open(filename3, 'rb') as f3:
    for i in f1:
        j = f2.readline()
        k = f3.readline()
        print(i, j, 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, j, k in zip(files[0], files[1], files[2]):
        print(i, j, k)