Python for-loop超前

Question

我有一个Python for循环中,我需要向前看一个项目,看是否需要采取行动处理之前进行.

for line in file:
    if the start of the next line == "0":
        perform pre-processing
        ...
    continue with normal processing
    ...

有没有简单的方法在python中执行此操作？我目前的方法是将文件缓冲到一个数组,但这并不理想,因为文件相当大.

Answer 1

您可以使用此配方获取任何可迭代的预取下一个项目:

from itertools import tee, islice, izip_longest
def get_next(some_iterable, window=1):
    items, nexts = tee(some_iterable, 2)
    nexts = islice(nexts, window, None)
    return izip_longest(items, nexts)

用法示例:

for line, next_line in get_next(myfile):
    if next_line and next_line.startswith("0"):
        ... do stuff

window如果您想要查看2行或更多行,则代码允许您将参数作为较大的值传递.

Answer 2

您可以拥有prev_line存储前一行的位置,并在您根据条件阅读行时进行处理.

就像是:

prev_line = None
for line in file:
    if prev_line is not None and the start of the next line == "0":
        perform pre-processing on prev_line
        ...
    continue with normal processing
    ...
    prev_line = line

如有必要,您可能需要对最后一行进行其他处理,具体取决于您的逻辑.

Answer 3

按照nosklo的回答,我倾向于使用以下模式:

该功能pairwise从优秀itertools食谱是非常理想的:

from itertools import tee

def pairwise(iterable):
    "s -> (s0,s1), (s1,s2), (s2, s3), ..."
    a, b = tee(iterable)
    next(b, None)
    return izip(a, b)

在代码中使用它可以获得:

for line, next_line in pairwise(file):
    if next_line.startswith("0"):
        pass #perform pre-processing
        #...
    pass #continue with normal processing

通常,对于这种类型的处理(可迭代中的前瞻),我倾向于使用窗口函数.Pairwise是一个大小为2的窗口的特例.