python - 如何进行从 k 到给定数字并返回到零的无限循环？

Question

我想要类似的东西(如果从2开始):

2, 3, ..., n-1, n, n-1, ..., 3, 2, 1, 0, 1, 2, 3, ...(永远)

还有比这更简单的吗？

def print_numbers_forth_and_back_forever(first_number, upper_limit):
    for i in range(first_number, upper_limit):
        print(i)
    while True:
        # back from n to 0
        for i in reversed(range(0, upper_limit+1)):
            print(i)
        # 1 to n-1
        for i in range(1, upper_limit):
            print(i)


print_numbers_forth_and_back_forever(4, 10)

Answer 1

以下使用itertools将提供一个应满足要求的生成器 - 只需更改range(a, b)的参数即可更改输出:

from itertools import cycle, chain

r = range(10)
r_reversed = reversed(r[1:-1])
gen = cycle(chain(r, r_reversed))

输出:

>>> from itertools import islice
>>> list(islice(gen, 20))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 1]

要从任意整数开始，请使用 islice 的 start 参数，如下所示:

>>> list(islice(gen, 4, 20))
[4, 5, 6, 7, 8, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 1]