使用 Python 遍历文件时请参考上一行

Question

我有一个如下所示的循环：

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

但我不想在每次迭代时只打印当前行，我还想打印前一行，如下所示，但代码没有给我我正在寻找的内容：

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

Answer 1

迭代器只能前进，所以没有previous()功能。

只需将当前行存储在一个变量中；它将是下一次迭代的前一个：

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