itertools — 创建高效循环迭代器的函数 ¶

---

此模块实现了许多 iterator 构建块受 APL、Haskell 和 SML 构造所启发。每个被重新铸造成适合 Python 的形式。

此模块标准化了一组快速、内存高效的核心工具，通过它们自身或将它们组合在一起很有用。它们一起形成了 "迭代器代数"，使以纯 Python 简洁且高效地构造专用工具成为可能。

例如，SML 提供的制表工具： tabulate(f) 产生一系列 f(0), f(1), ... 。Python 可以达成相同效果通过组合 map() and count() 以形成 map(f, count()) .

These tools and their built-in counterparts also work well with the high-speed functions in the operator module. For example, the multiplication operator can be mapped across two vectors to form an efficient dot-product: sum(starmap(operator.mul, zip(vec1, vec2, strict=True))) .

无限迭代器：

迭代器	自变量	结果	范例
count()	[start[, step]]	start, start+step, start+2*step, …	count(10) → 10 11 12 13 14 ...
cycle()	p	p0, p1, … plast, p0, p1, …	cycle('ABCD') → A B C D A B C D ...
repeat()	elem [,n]	elem, elem, elem, … endlessly or up to n times	repeat(10, 3) → 10 10 10

Iterators terminating on the shortest input sequence:

迭代器	自变量	结果	范例
accumulate()	p [,func]	p0, p0+p1, p0+p1+p2, …	accumulate([1,2,3,4,5]) → 1 3 6 10 15
batched()	p, n	(p0, p1, …, p_n-1), …	batched('ABCDEFG', n=3) → ABC DEF G
chain()	p, q, …	p0, p1, … plast, q0, q1, …	chain('ABC', 'DEF') → A B C D E F
chain.from_iterable()	iterable	p0, p1, … plast, q0, q1, …	chain.from_iterable(['ABC', 'DEF']) → A B C D E F
compress()	data, selectors	(d[0] if s[0]), (d[1] if s[1]), …	compress('ABCDEF', [1,0,1,0,1,1]) → A C E F
dropwhile()	predicate, seq	seq[n], seq[n+1], starting when predicate fails	dropwhile(lambda x: x<5, [1,4,6,3,8]) → 6 3 8
filterfalse()	predicate, seq	elements of seq where predicate(elem) fails	filterfalse(lambda x: x<5, [1,4,6,3,8]) → 6 8
groupby()	iterable[, key]	sub-iterators grouped by value of key(v)	groupby(['A','B','DEF'], len) → (1, A B) (3, DEF)
islice()	seq, [start,] stop [, step]	elements from seq[start:stop:step]	islice('ABCDEFG', 2, None) → C D E F G
pairwise()	iterable	(p[0], p[1]), (p[1], p[2])	pairwise('ABCDEFG') → AB BC CD DE EF FG
starmap()	func, seq	func(*seq[0]), func(*seq[1]), …	starmap(pow, [(2,5), (3,2), (10,3)]) → 32 9 1000
takewhile()	predicate, seq	seq[0], seq[1], until predicate fails	takewhile(lambda x: x<5, [1,4,6,3,8]) → 1 4
tee()	it, n	it1, it2, … itn splits one iterator into n
zip_longest()	p, q, …	(p[0], q[0]), (p[1], q[1]), …	zip_longest('ABCD', 'xy', fillvalue='-') → Ax By C- D-

组合迭代器：

迭代器	自变量	结果
product()	p, q, … [repeat=1]	cartesian product, equivalent to a nested for-loop
permutations()	p[, r]	r-length tuples, all possible orderings, no repeated elements
combinations()	p, r	r-length tuples, in sorted order, no repeated elements
combinations_with_replacement()	p, r	r-length tuples, in sorted order, with repeated elements

范例	结果
product('ABCD', repeat=2)	AA AB AC AD BA BB BC BD CA CB CC CD DA DB DC DD
permutations('ABCD', 2)	AB AC AD BA BC BD CA CB CD DA DB DC
combinations('ABCD', 2)	AB AC AD BC BD CD
combinations_with_replacement('ABCD', 2)	AA AB AC AD BB BC BD CC CD DD

Itertool Functions ¶

The following functions all construct and return iterators. Some provide streams of infinite length, so they should only be accessed by functions or loops that truncate the stream.

itertools. accumulate ( iterable [ , function , * , initial=None ] ) ¶

Make an iterator that returns accumulated sums or accumulated results from other binary functions.

The function defaults to addition. The function should accept two arguments, an accumulated total and a value from the iterable .

若 initial value is provided, the accumulation will start with that value and the output will have one more element than the input iterable.

大致相当于：

def accumulate(iterable, function=operator.add, *, initial=None):
    'Return running totals'
    # accumulate([1,2,3,4,5]) → 1 3 6 10 15
    # accumulate([1,2,3,4,5], initial=100) → 100 101 103 106 110 115
    # accumulate([1,2,3,4,5], operator.mul) → 1 2 6 24 120
    iterator = iter(iterable)
    total = initial
    if initial is None:
        try:
            total = next(iterator)
        except StopIteration:
            return
    yield total
    for element in iterator:
        total = function(total, element)
        yield total