Python for-loop look-ahead

Along the lines of nosklo's answer, I tend to use the following pattern:

The function pairwise from the excellent itertools recipes is ideal for this:

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)

Using it in your code gets us:

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

Generally, for this type of processing (lookahead in the iterable), I tend to use a window function. Pairwise is a special case of a window of size 2.

you can get any iterable to prefetch next item with this recipe:

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)

Example usage:

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

The code allows you to pass the window parameter as a larger value, if you want to look 2 or more lines ahead.