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')
  # 输出: ouoeaeeyeaao
  print just_vowels(u'tiger,tiger burning bright')
  # 输出: ieieuii