使用Python遍历文件时，请参考上一行

Question

我有一个如下的循环：

with open(file, 'r') as f:
    next(f)
    for line in f:
        print line

但我不想每次迭代都只打印当前行，我还想打印下面的前一行，但代码并没有给出我要查找的内容：

with open(file, 'r') as f:
    next(f)
    for line in f:
        previous(f)    #this does not go to the previous line
        print line
        next(f)
        print line
        next(f)

结果应该是这样的：

输入：

line1
line2
line3

输出：

line1
line2
line2
line3