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 = 1
for item in count():
print item,
if item > 5:
break
print
print "-"*20
#指定参数，start = 10, step = 2
for 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 22itertools.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 cycle
l = ['a', 'b', 'c']
i = 0
for item in cycle(l):
print item,
i += 1
if i > 4:
break
#输出结果为：
a b c a bitertools.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 itertools
l = [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]