>>> f_set = frozenset("qiwsir")   #看這個名字就知道了frozen，凍結的set
>>> f_set
frozenset(['q', 'i', 's', 'r', 'w'])
>>> f_set.add("python")       #報錯
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: 'frozenset' object has no attribute 'add'

>>> a_set = set("github")      #對比看一看，這是一個可以原處修改的set
>>> a_set
set(['b', 'g', 'i', 'h', 'u', 't'])
>>> a_set.add("python")
>>> a_set
set(['b', 'g', 'i', 'h', 'python', 'u', 't'])

>>> aset
set(['h', 'o', 'n', 'p', 't', 'y'])
>>> "a" in aset
False
>>> "h" in aset
True

>>> a      
set(['q', 'i', 's', 'r', 'w'])
>>> b
set(['a', 'q', 'i', 'l', 'o'])
>>> a == b
False
>>> a != b
True

>>> a
set(['q', 'i', 's', 'r', 'w'])
>>> c
set(['q', 'i'])
>>> c<a   #c是a的子集
True
>>> c.issubset(a)  #或者用這種方法，判斷c是否是a的子集
True
>>> a.issuperset(c) #判斷a是否是c的超集
True

>>> b
set(['a', 'q', 'i', 'l', 'o'])
>>> a<b   #a不是b的子集
False
>>> a.issubset(b)  #或者這樣做
False

>>> a
set(['q', 'i', 's', 'r', 'w'])
>>> b
set(['a', 'q', 'i', 'l', 'o'])
>>> a | b            #可以有兩種方式，結果一樣
set(['a', 'i', 'l', 'o', 'q', 's', 'r', 'w'])
>>> a.union(b)
set(['a', 'i', 'l', 'o', 'q', 's', 'r', 'w'])

>>> a
set(['q', 'i', 's', 'r', 'w'])
>>> b
set(['a', 'q', 'i', 'l', 'o'])
>>> a & b    #兩種方式，等價
set(['q', 'i'])
>>> a.intersection(b)
set(['q', 'i'])

>>> a and b
set(['a', 'q', 'i', 'l', 'o'])

>>> a
set(['q', 'i', 's', 'r', 'w'])
>>> b
set(['a', 'q', 'i', 'l', 'o'])
>>> a - b
set(['s', 'r', 'w'])
>>> a.difference(b)
set(['s', 'r', 'w'])

>>> a
set(['q', 'i', 's', 'r', 'w'])
>>> b
set(['a', 'q', 'i', 'l', 'o'])
>>> a.symmetric_difference(b)
set(['a', 'l', 'o', 's', 'r', 'w'])