集合

>>> set1 = [1,1,1,2,4,4,6,7,5,1,8,6,9]
>>> set2 = set(set1)
>>> set2
{1, 2, 4, 5, 6, 7, 8, 9}

>>> set3 = frozenset(set2)
>>> set3.add(12)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'frozenset' object has no attribute 'add'