docs: def extract_keys(self,key_list):
          """ >>> d = {'a':1,'b':2,'c':3}
              >>> print d.extract_keys('b,c,d')
              >>> {'b':2,'c':3}
              >>> print d.extract_keys(['b','c','d'])
              >>> {'b':2,'c':3} """
          if isinstance(key_list,basestring):
              key_list = key_list.split(',')
          return type(self)([ (k,self[k]) for k in key_list if k in self ])