内建函数-frozenset
frozenset：元素一经创建，不可增加、删除和修改。因此没有add、pop、remove的方法。但可以作为左值接受赋值。
可以通过如下方法查看set和frozenset的区别
frozen_set = frozenset((1,2,3,4))
print(dir(frozen_set))
print('---------------------------------------')
normal_set = set((1,2,3,4))
print(dir(normal_set))
输出为：
['__and__', '__class__', '__contains__', '__delattr__', '__dir__', '__doc__', '_ _eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__ini t__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__or__', '__rand__', '__reduce__', '__reduce_ex__', '__repr__', '__ror__', '__rsub__', '_ _rxor__', '__setattr__', '__sizeof__', '__str__', '__sub__', '__subclasshook__', '__xor__', 'copy', 'difference', 'intersection', 'isdisjoint', 'issubset', 'iss uperset', 'symmetric_difference', 'union'] --------------------------------------- ['__and__', '__class__', '__contains__', '__delattr__', '__dir__', '__doc__', '_ _eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__ian d__', '__init__', '__ior__', '__isub__', '__iter__', '__ixor__', '__le__', '__le n__', '__lt__', '__ne__', '__new__', '__or__', '__rand__', '__reduce__', '__redu ce_ex__', '__repr__', '__ror__', '__rsub__', '__rxor__', '__setattr__', '__sizeo f__', '__str__', '__sub__', '__subclasshook__', '__xor__', 'add', 'clear', 'copy ', 'difference', 'difference_update', 'discard', 'intersection', 'intersection_u pdate', 'isdisjoint', 'issubset', 'issuperset', 'pop', 'remove', 'symmetric_diff erence', 'symmetric_difference_update', 'union', 'update']
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