python的set 操作

set is an unordered collection of elements,is a map in which keys do not have associated values. 
python have builtins frozenset and set.frozenset is an immutable form. these are implemented using hash table.

from collections import MutableSet                #set
from collections import Set                        #immutable set

基本操作：

some_set={7,2,3,5}
some_set.add(6)                #add the item
some_set.update({10,9})        #add new set to this set
some_set.remove(6)            #remove if the item exist, or raise an KeyError Exception.
some_set.discard(6)            #discard an item and no exception if it does not exist.
some_set.pop()                #return an arbitrary item, raise KeyError if set is empty.
some_set.clear()            #清空一个集合

sorted(some_set)
max(some_set)
min(some_set)
sum(some_set)
words=set('How I wish'.split())
more=set('I would recollect pi'.split())
words|more                #words.union(more)                 并集
words&more                #words.intersection(more)        交集
words|=more                #words.update(more)                把第二个集合中不在第一个集合中的数据加入到第一个集合
words-more                #words.difference(more)            第一个集合减去两个集合的交集
words^more                #words.symmetric_difference(more)    剪去两个集合的交集，并把第二个集合中特有的元素加入第一个集合
{1,2,3}.issubset({1,2,3,4,5,6})        #第一个集合是第二个集合的子集
{1,2,3,4,5}.issubset({1,2,3})        #False
{1,2,3}.issuperset({1,2,3,4,5,6})    #False
{1,2,3,4,5,6}.issuperset({1,2,3})    #第一个集合是第二个集合的超集
not({1,2,3}-{1,2,3,4,5})            #True,空集合为False
not({1,2,3,4,5}-{1,2,3})            #False

'I' in words            #True
{'I'} < words            #True
{'How','I','wish'} <= words        #True
S|T: | 是用__or__实现的
S|=T：是__ior__ 实现
表示空集合用 set() ,最好不用{}