一、chain / chain.from_iterable
二、izip / imap / starmap
三、count / cycle / repeat
四、ifilter/ islice
五、compress / dropwhile / takewhile
六、groupby
七、produce / permutations / combinations
参考：
[1] https://docs.python.org/2.7/library/itertools.html#module-itertools
[2] https://pymotw.com/2/itertools/index.html
