itertools.chain()的广度优先版本

问题描述

在itertools中有一个chain，它将多个生成器组合到一个生成器中，并且实质上对它们进行了深度优先迭代，即chain.from_iterable(['ABC', '123'])生成A，B，C，1、2、3但是，没有广度优先版本，还是我错过了什么?当然有izip_longest，但是对于大量的生成器来说，这感觉很尴尬，因为元组会很长，甚至很稀疏.

In itertools there's chain, which combines multiple generators in a single one, and in essence does a depth-first iteration over them, i.e., chain.from_iterable(['ABC', '123']) yields A, B, C, 1, 2, 3. But, there's no breadth-first version, or am I missing something? There's of course izip_longest, but for large numbers of generators this feels awkward, as the tuples will be very long and possibly very sparse.

我想出了以下几点:

def chain_bfs(*generators):
    generators = list(generators)
    while generators:
        g = generators.pop(0)
        try:
            yield g.next()
        except StopIteration:
            pass
        else:
            generators.append(g)

对我来说，这有点冗长，我还想念一种更Python化的方法吗?并且此函数是否可以很好地包含在itertools中?

It feels a bit verbose to me, is there a more Pythonic approach I'm missing? And would this function be a good candidate for inclusion in itertools?

推荐答案

您可以使用 遍历迭代器；旋转双端队列效率更高.我也将其称为链式拉链，而不是呼吸优先链"，例如:

You could use collections.deque() to rotate through your iterators; rotating a deque is much more efficient. I'd also call it a chained zip, not a 'breath first chain', as such:

from collections import deque

def chained_zip(*iterables):
    iterables = deque(map(iter, iterables))
    while iterables:
        try:
            yield next(iterables[0])
        except StopIteration:
            iterables.popleft()
        else:
            iterables.rotate(-1)

演示:

>>> list(chained_zip('ABC', '123'))
['A', '1', 'B', '2', 'C', '3']
>>> list(chained_zip('AB', '1234'))
['A', '1', 'B', '2', '3', '4']

文档中还有一个 roundrobin()配方 ，使用 itertools.cycle()函数:

There is also a roundrobin() recipe in the documentation that does the same, using the itertools.cycle() function:

def roundrobin(*iterables):
    "roundrobin('ABC', 'D', 'EF') --> A D E B F C"
    # Recipe credited to George Sakkis
    pending = len(iterables)
    nexts = cycle(iter(it).__next__ for it in iterables)
    while pending:
        try:
            for next in nexts:
                yield next()
        except StopIteration:
            pending -= 1
            nexts = cycle(islice(nexts, pending))

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