案例：统计元素在列表出现的频次
方案1
from random import randint
data = [randint(0,20) for _ in xrange(20)]
c = dict.fromkeys(data,0)
for v in data:
c[v] += 1
tmp = sorted(c.values(),reverse = True)
[tmp[i] for i in xrange(3)]
方案2
# 使用collections 总的Counter
from collections import Counter
Counter(re.split('\W+',open('/etc/profile').read())).most_common(3)
