groupby()可對(duì)list中的元素進(jìn)行分組，并統(tǒng)計(jì)連續(xù)相同元素出現(xiàn)的次數(shù)
（*非連續(xù)的元素重新分組、統(tǒng)計(jì)）

import itertools
a = ['a', 'b', 'b', 'c', 'd', 'd', 'd', 'a', 'a']
items, counts = [], []
for k, v in itertools.groupby(a):
   items.append(k)
   counts.append(len(list(v)))
print 'items:', items
print 'counts:', counts
>>>
items: ['a', 'b', 'c', 'd', 'a']
counts: [1, 2, 1, 3, 2]