Python:在到达结尾时循环切换列表中的元素

Question

我有一个看起来像这样的列表:

a = ['01', '02', '03', '04', '05', '06', '07', '08', '09', '10']

我需要一次循环遍历此列表中的一个元素,但是当到达列表的末尾时,需要反转循环.

例如,使用itertools.cycle:

from itertools import cycle
a_cycle = cycle(a)
for _ in range(30):
    print a_cycle.next()

我明白了:

01, 02, 03, 04, 05, 06, 07, 08, 09, 10, 01, 02, 03, 04, 05, 06, 07, 08, 09, 10, 01, 02, 03, 04, 05, 06, 07, 08, 09, 10

但我需要的是:

01, 02, 03, 04, 05, 06, 07, 08, 09, 10, 10, 09, 08, 07, 06, 05, 04, 03, 02, 01, 01, 02, 03, 04, 05, 06, 07, 08, 09, 10

我需要循环a一段固定的次数,比如200.

Answer 1

你可以cycle在chain你的a和,例如:reversed a

from itertools import cycle, islice, chain

a = range(1, 11)
b = reversed(a)
c = cycle(chain(a, b))
d = list(islice(c, 100)) # `c` is infinite - hence the `islice` to stop at some point...

哪个给你:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

注意:如果a是可耗尽的迭代器,则需要先复制一份a.但是举个例子,这很好.