摘要：并且集合中的元素也是隨機排序的。除了用來創建集合，還可以使用的方式，但是這種方式不提倡使用，因為在某些情況下，搞不清楚是字典還是集合。集合不是序列類型，不能用索引方式對其進行修改和實現集合和列表兩種對象之間的轉化。
集合Set類似字典的特點，可以用{}花括號來定義；其中的元素是沒有序列，也就是非序列類型的數據；而且集合中的元素不可重復，這就類似于dict鍵。
創建集合
>>> s1 = set("qiswri") #有兩個i
>>> s1
set(["q", "i", "s", "r", "w"]) #只有一個i
>>>
>>> s2 = set([123,"google","facebook","book","facebook"]);
>>> s2
set(["facebook", 123, "google", "book"]) #只有一個facebook
說明集合中的元素是不能重復的，在創建集合的時候，如果發現了重復的元素，就會自定過濾重復的元素。并且集合中的元素也是隨機排序的。
除了用set()來創建集合，還可以使用{}的方式，但是這種方式不提倡使用，因為在某些情況下，Python搞不清楚是字典還是集合。
unhashtable（不可哈希的）表示可變數據，如列表和字典都能原地修改，就是unhastable的；
hastable（可哈希）表示不可變數據，如字符串不能修改。
>>> s1
set(["q", "i", "s", "r", "w"])
>>> s1[1] = "w" #集合不是序列類型，不能用索引方式對其進行修改
Traceback (most recent call last):
File "", line 1, in
s1[1] = "w"
TypeError: "set" object does not support item assignment
list()和set()實現集合和列表兩種對象之間的轉化。
>>> s2
set(["facebook", 123, "google", "book"])
>>> type(s2)
>>> lst = list(s2)
>>> lst
["facebook", 123, "google", "book"]
>>> type(lst)
>>> s3 = set(lst)
>>> s3
set(["google", 123, "facebook", "book"])
集合的函數
dir(set)：列出集合的函數
add和update
help(set.add)
>>> s1 = set(["a","b","c"])
>>> s1
set(["a", "c", "b"])
>>> s1.add("abc") #向集合中添加元素
>>> s1
set(["a", "c", "b", "abc"])
>>> s1.add([1,2,3]) #不能添加列表，因為列表的數據是可變的
Traceback (most recent call last):
File "", line 1, in
s1.add([1,2,3])
TypeError: unhashable type: "list"
>>>
>>> s1
set(["a", "c", "b", "abc"])
>>> s2 = set(["e","f","g"])
>>> s1.update(s2) #集合s2添加到s1中
>>> s1
set(["a", "c", "b", "e", "abc", "g", "f"])
>>> s2
set(["e", "g", "f"])
pop、remove、discard、clear
>>> s1
set(["a", "c", "b", "e", "abc", "g", "f"])
>>> s1.pop() #隨機刪除一個元素，并返回這個元素
"a"
>>> s1.pop()
"c"
>>> s1.pop("b") #不能指定參數
Traceback (most recent call last):
File "", line 1, in
s1.pop("b")
TypeError: pop() takes no arguments (1 given)
>>>
>>> s1.remove("b") #刪除集合中指定的元素
>>> s1
set(["e", "abc", "g", "f"])
>>> s1.remove() #remove必須指定參數
Traceback (most recent call last):
File "", line 1, in
s1.remove()
TypeError: remove() takes exactly one argument (0 given)
>>> s1.discard("g") #存在則刪除集合中的元素
>>> s1
set(["e", "abc", "f"])
>>> s1.discard("i") #不存在則不做處理，不拋出異常
>>>
>>> s1.clear() #清空集合
>>> s1
set([])
不變的集合
>>> f1 = frozenset("qwert")
>>> f1
frozenset(["q", "r", "e", "t", "w"])
>>> f1.add("q") #不能修改集合
Traceback (most recent call last):
File "", line 1, in
f1.add("q")
AttributeError: "frozenset" object has no attribute "add"
>>> f1.remove("r")
Traceback (most recent call last):
File "", line 1, in
f1.remove("r")
AttributeError: "frozenset" object has no attribute "remove"
>>> f1.pop()
Traceback (most recent call last):
File "", line 1, in
f1.pop()
AttributeError: "frozenset" object has no attribute "pop"
集合元素
元素與集合的關系
>>> aset=set(["q","w","e","r","t"])
>>> aset
set(["q", "r", "e", "t", "w"])
>>> "q" in aset
True
>>> "a" in aset
False
集合與集合的關系
A是否等于B
>>> a
set(["q", "r", "e", "t", "w"])
>>> b = set(["a","b","c"])
>>> a == b
False
>>> a != b
True
A是否是B的子集
>>> c = set(["q","r"])
>>> a
set(["q", "r", "e", "t", "w"])
>>>
>>> c.issubset(a) #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
AB的并集
>>> a
set(["q", "r", "e", "t", "w"])
>>> b
set(["a", "c", "b"])
>>> a | b
set(["a", "c", "b", "e", "q", "r", "t", "w"]) #ab的并集
>>> a.union(b)
set(["a", "c", "b", "e", "q", "r", "t", "w"]) #ab的并集
AB的交集
>>> a
set(["q", "r", "e", "t", "w"])
>>> b
set(["a", "q", "c", "b"])
>>> a & b #ab的交集
set(["q"])
>>> a.intersection(b)
set(["q"])
A相對B的差集
A相對B不同的部分元素集合
>>> a
set(["q", "r", "e", "t", "w"])
>>> b
set(["a", "q", "c", "b"])
>>> a - b
set(["r", "e", "t", "w"])
>>> a.difference(b)
set(["r", "e", "t", "w"])
AB的差集
AB中不同元素的集合
>>> a
set(["q", "r", "e", "t", "w"])
>>> b
set(["a", "q", "c", "b"])
>>> a.symmetric_difference(b)
set(["a", "c", "b", "e", "r", "t", "w"])
