摘要：并且集合中的元素也是隨機(jī)排序的。除了用來(lái)創(chuàng)建集合，還可以使用的方式，但是這種方式不提倡使用，因?yàn)樵谀承┣闆r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(duì)其進(jìn)行修改和實(shí)現(xiàn)集合和列表兩種對(duì)象之間的轉(zhuǎn)化。
集合Set類似字典的特點(diǎn)，可以用{}花括號(hào)來(lái)定義；其中的元素是沒(méi)有序列，也就是非序列類型的數(shù)據(jù)；而且集合中的元素不可重復(fù)，這就類似于dict鍵。
創(chuàng)建集合
>>> s1 = set("qiswri") #有兩個(gè)i
>>> s1
set(["q", "i", "s", "r", "w"]) #只有一個(gè)i
>>>
>>> s2 = set([123,"google","facebook","book","facebook"]);
>>> s2
set(["facebook", 123, "google", "book"]) #只有一個(gè)facebook
說(shuō)明集合中的元素是不能重復(fù)的，在創(chuàng)建集合的時(shí)候，如果發(fā)現(xiàn)了重復(fù)的元素，就會(huì)自定過(guò)濾重復(fù)的元素。并且集合中的元素也是隨機(jī)排序的。
除了用set()來(lái)創(chuàng)建集合，還可以使用{}的方式，但是這種方式不提倡使用，因?yàn)樵谀承┣闆r下，Python搞不清楚是字典還是集合。
unhashtable（不可哈希的）表示可變數(shù)據(jù)，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(shù)據(jù)，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(duì)其進(jìn)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(shí)現(xiàn)集合和列表兩種對(duì)象之間的轉(zhuǎn)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(shù)
dir(set)：列出集合的函數(shù)
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?yàn)榱斜淼臄?shù)據(jù)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(jī)刪除一個(gè)元素，并返回這個(gè)元素
"a"
>>> s1.pop()
"c"
>>> s1.pop("b") #不能指定參數(shù)
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(shù)
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(guān)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(guān)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(duì)B的差集
A相對(duì)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"])
