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python itertools循环解释

iterator python

xuanyue · 技术社区 · 6 年前

我在 https://more-itertools.readthedocs.io/en/latest/api.html ,但我不明白最后一行 nexts = cycle(islice(nexts, pending)) 从nexts迭代器中移除耗尽的生成器。据我所知,这会从输入中删除最后一个,但我们不应该删除第一个,即引发此异常的当前那个吗?

def roundrobin(*iterables):
    """Yields an item from each iterable, alternating between them.

        >>> list(roundrobin('ABC', 'D', 'EF'))
        ['A', 'D', 'E', 'B', 'F', 'C']

    This function produces the same output as :func:`interleave_longest`, but
    may perform better for some inputs (in particular when the number of
    iterables is small).

    """
    # Recipe credited to George Sakkis
    pending = len(iterables)
    if PY2:
        nexts = cycle(iter(it).next for it in iterables)
    else:
        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))

2 回复 | 直到 6 年前

Taku 6 年前

我会一步一步地为你拆散它:

>>> list(roundrobin('ABC', 'D', 'EF'))


pending = 3  # len(('ABC', 'D', 'EF'))

# nexts roughly equivalent to:
['ABC', 'D', 'EF', 'ABC', 'D', 'EF', 'ABC', ...]

***

# the following get yielded 
'A' from 'ABC' # nexts: ['D' , 'EF', 'BC', 'D' , 'EF', ...]
'D' from 'D'   # nexts: ['EF', 'BC', ''  , 'EF', 'BC', ...]
'E' from 'EF'  # nexts: ['BC', ''  , 'F' , 'BC', ''  , ...]
'B' from 'BC'  # nexts: [''  , 'F' , 'C' , ''  , 'F' , ...]

# StopIteration was raised by what used to be "B"
SI from ''  # nexts:  ['F', 'C', '', 'F', 'C', '', 'F', ...]
#                                 ^ index 2 (`pending`[see the following line])

# pending -= 1
pending = 2

# when islice() is used with one argument, it defaults to the "stop" index
# so islice() returns ['F', 'C']
# and cycle() converts it to ['F', 'C', 'F', 'C', ...]

pending = 2
nexts: ['F', 'C', 'F', 'C', ...]

# Go back to *** and continue until pending = 0

这就是最后一行如何删除耗尽的iterable。

主要思想:

for next in nexts:
    yield next()

即使 StopIteration 被养大,精疲力尽的衣食住行已经从 nexts .

因此,与其将耗尽的iterable作为 NEXTS :

nexts = ['', 'F', 'C', '', 'F', 'C', '', 'F', ...]

第一项 NEXTS 实际上是下一个项目:

nexts = ['F', 'C', '', 'F', 'C', '', 'F', ...]

Brad Solomon 6 年前

因为 cycle() 创建一个无限迭代器,并不总是从 iterables 什么时候? islice() 在最后一行中使用。简化一点, nexts 因为它最初的定义将“循环回”到 迭代函数 :

try:
    for next in nexts:
        print(next())
except StopIteration:
    print('decrementing...')
A
D
E
B
decrementing...

此时的目标是移除 'D' . 关键是你现在“在” 'EF' ,使用迭代器 'ABC' 排在第二,所以 islice(nexts, pending) 将从 “EF” 到 “ABC” .

下面是一个相关的例子:

x = cycle('abcd')
#          ^  |
#          |__|

for _ in range(3):
    x.__next__()

list(islice(x, 2))
# ['d', 'a']