python模块之itertools

简介

NAME    itertools - Functional tools for creating and using iterators.FILE    (built-in)DESCRIPTION    Infinite iterators:    count([n]) --> n, n+1, n+2, ...    cycle(p) --> p0, p1, ... plast, p0, p1, ...    repeat(elem [,n]) --> elem, elem, elem, ... endlessly or up to n times    Iterators terminating on the shortest input sequence:    chain(p, q, ...) --> p0, p1, ... plast, q0, q1, ...     compress(data, selectors) --> (d[0] if s[0]), (d[1] if s[1]), ...    dropwhile(pred, seq) --> seq[n], seq[n+1], starting when pred fails    groupby(iterable[, keyfunc]) --> sub-iterators grouped by value of keyfunc(v)    ifilter(pred, seq) --> elements of seq where pred(elem) is True    ifilterfalse(pred, seq) --> elements of seq where pred(elem) is False    islice(seq, [start,] stop [, step]) --> elements from           seq[start:stop:step]    imap(fun, p, q, ...) --> fun(p0, q0), fun(p1, q1), ...    starmap(fun, seq) --> fun(*seq[0]), fun(*seq[1]), ...    tee(it, n=2) --> (it1, it2 , ... itn) splits one iterator into n    takewhile(pred, seq) --> seq[0], seq[1], until pred fails    izip(p, q, ...) --> (p[0], q[0]), (p[1], q[1]), ...     izip_longest(p, q, ...) --> (p[0], q[0]), (p[1], q[1]), ...     Combinatoric generators:    product(p, q, ... [repeat=1]) --> cartesian product    permutations(p[, r])    combinations(p, r)    combinations_with_replacement(p, r)

itertools. count

count(start=0, step=1) –> count object
Return a count object whose .next() method returns consecutive values.
在官方文档里面，有一段代码，解释说，count函数等价与下面的函数
Equivalent to:

def count(firstval=0, step=1):    x = firstval    while 1:        yield x        x += step

因此，count函数两个参数开始值和步长均有默认值，其返回一无界的迭代器，在循环里面调用的时候，要设置break条件。

#!/usr/bin/env python#coding:utf8"""Author:Description: learning itertools.count"""from  itertools import count#使用默认参数，start = 0, step = 1for item in count():    print item,    if item > 5:        breakprintprint "-"*20#指定参数，start = 10, step = 2for item in count(start = 10, step = 2):    print item,    if item > 20:        break#输出结果为：0 1 2 3 4 5 6--------------------10 12 14 16 18 20 22

itertools.cycle

cycle(iterable) –> cycle object
Return elements from the iterable until it is exhausted.Then repeat the sequence indefinitely.
一个列表为例，返回每一个元素，直到列表的最后一个元素，然后重复返回这个列表，无限循环下去, 也是返回一个无界迭代器。
直接上代码吧：

#!/usr/bin/env python#coding:utf8"""Author:Description: learning itertools.cycle"""from  itertools import cyclel = ['a', 'b', 'c']i = 0for item in cycle(l):    print item,    i += 1    if i > 4:        break#输出结果为：a b c a b

itertools.repeat

repeat(object [,times]) -> create an iterator which returns the object
for the specified number of times. If not specified, returns the object endlessly.
返回一个迭代器，可以指定重复的次数。若不指定次数，则是返回一个无界的迭代器。

import itertoolsl = [1, 2, 3]for item in itertools.repeat(l, 3):    print l,# 输出结果为：   [1, 2, 3] [1, 2, 3] [1, 2, 3]