python过滤字符串中不属于指定集合中字符的类实例

# -*- coding: utf-8 -*-
import sets
class Keeper(object):
 def __init__(self, keep):
 self.keep = sets.Set(map(ord, keep))
 def __getitem__(self, n):
 if n not in self.keep:
 return None
 return unichr(n)
 def __call__(self, s):
 return s.translate(self)
makefilter = Keeper
if __name__ == '__main__':
 just_vowels = makefilter('aeiouy')
 print just_vowels(u'four score and seven years ago')
 #