python 中的set与list,tuple

 __author__ = 'liunnis'
 #-*-coding:utf-8 -*-
 a=[1,2,3,4,4]
 print a
 print list(set(a))
 b=[str(i) for i in a]
 print list(set(b))
 #the result is
 # [1, 2, 3, 4, 4]
 # [1, 2, 3, 4]
 # ['1', '3', '2', '4']
 c=[[1,2],[3,4],[5,5],[6,6],[7,8],[9,0],10,[11]]
 print c
 try:
     print set(c)
 except TypeError,e:
     print'Error:',e
 #the result is
 # [[1, 2], [3, 4], [5, 5], [6, 6], [7, 8], [9, 0], 10, [11]]
 # Error: unhashable type: 'list'