python - 如何在指定起始索引的同时无限迭代列表？

Question

我有一个名为 all_urls 的列表。我正在尝试无限迭代该列表。我有一个索引 i 我想从它开始。例如:

www.google.com

www.facebook.com

www.linkednin.com

www.yahoo.com

www.microsoft.com

我想从 linkedin 开始迭代，然后进入无限循环。

我尝试查看此链接:

How can I infinitely loop an iterator in Python, via a generator or other?

但此链接没有指定我如何从某个中间点开始。

我已经尝试过

    for url in itertools.cycle(all_urls[i:]):
        print "Process URL : " + url

但它不起作用。

Answer 1

您有几个选择。也许最简单的选择是构建一个新的循环列表:

urls = all_urls[i:] + all_urls[:i]
for url in itertools.cycle(urls):
   print(url)

我提出的第二个解决方案是仅使用循环的前 i - 1 个元素:

# From https://docs.python.org/2/library/itertools.html#recipes
def consume(iterator, n):
    "Advance the iterator n-steps ahead. If n is none, consume entirely."
    # Use functions that consume iterators at C speed.
    if n is None:
        # feed the entire iterator into a zero-length deque
        collections.deque(iterator, maxlen=0)
    else:
        # advance to the empty slice starting at position n
        next(itertools.islice(iterator, n, n), None)

urls = itertools.cycle(all_urls)
consume(urls, i - 1)
for url in urls:
    print(url)