Python版本：3.6.2 操作系统：Windows 作者：SmallWZQ
经过几天的回顾和学习，我终于把Python 3.x中的基础知识介绍好啦。下面将要继续什么呢？让我想想先~~~嗯，还是先整理一下近期有关Python基础知识的随笔吧。
Python编程软件的安装与使用——Windows、Linux和Mac
Python基础——输出[print()]与输入[input()]
上述六篇均为Python 3.x的基础知识。九尺高台，起于累土。学习Python就要从最基本开始，经过逐步的积累，才能有所成就。
Python基础知识再次回顾好了，接下来该干嘛呢？这不继续Python数据结构了吗？
上次我写了有关Python数据结构（列表、元组、字典）的3篇随笔：
本篇随笔将开始一段关于set（集合）之旅吧。
什么是集合呢？
说到集合，我首先想到了高中的数学。高中，人生学习中最繁忙的一段时光。直到现在，我能回忆起最多的就是学习、学习、还是读书……言归正传，高一时的数学，我们就接触到了集合。书中应该是这样定义的：
集合：由一个或多个确定的元素所构成的整体。若x是集合A的元素，则记作x∈A。
集合中的元素有三个特征：
1. 确定性：集合中的元素必须是确定的；
2. 互异性：集合中的元素互不相同，例如：集合A={1，a}，则a不能等于1）；
3. 无序性：集合中的元素没有先后之分，例如：集合{3,4,5}和{3,5,4}算作同一个集合。
Python 3.x中的set特征与数学中类似。我们之前学过list、tuple以及dict。其实，set与dict大致相同，但set没有Value，只有key。因此，set只是一组key的集合。由于key不能重复，所以，在set中，没有重复的key。
创建集合
1.1 创建空集合
在集合中，创建空集合（set）必须使用函数set()。
#创建空集合
>>>a = set()
>>>a
set()
>>>type(a)
<class 'set'>
注：不能使用{}，{}用于创建空字典。
1.2 创建非空集合
非空集合可以用大括号{}或 set()函数来创建。
#创建集合
>>>a={'a','b','c','d'}
>>>b=set('abcdefabcd')
>>>c=set({'a':1,'b':2,'c':3})
>>>d=set(['a','b','c','a'])
#运行结果
>>>print(a,type(a))
{'c', 'd', 'b', 'a'} <class 'set'>
>>>print(b,type(b))
{'f', 'e', 'b', 'c', 'd', 'a'} <class 'set'>
>>>print(c,type(c))
{'b', 'a','c'} <class 'set'>
>>>print(d,type(d))
{'c', 'b', 'a'} <class 'set'>
特别地，set中的元素是无序的，并且重复元素在set中自动被过滤。
#set中重复元素被自动过滤
>>>s = {1,2,,1,2,4,4,3,3}
>>>s
{1,2,3,4}
功能属性
set有很多很多的功能属性。你们不信？不信的话，继续往下看呗~~~
set功能属性如下：
1 class set(object):
2 """
3 set() -> new empty set object
4 set(iterable) -> new set object
5
6 Build an unordered collection of unique elements.
7 """
8 def add(self, *args, **kwargs): # real signature unknown
9 """
10 Add an element to a set.
11
12 This has no effect if the element is already present.
13 """
14 pass
15
16 def clear(self, *args, **kwargs): # real signature unknown
17 """ Remove all elements from this set. """
18 pass
19
20 def copy(self, *args, **kwargs): # real signature unknown
21 """ Return a shallow copy of a set. """
22 pass
23
24 def difference(self, *args, **kwargs): # real signature unknown
25 """
26 Return the difference of two or more sets as a new set.
27
28 (i.e. all elements that are in this set but not the others.)
29 """
30 pass
31
32 def difference_update(self, *args, **kwargs): # real signature unknown
33 """ Remove all elements of another set from this set. """
34 pass
35
36 def discard(self, *args, **kwargs): # real signature unknown
37 """
38 Remove an element from a set if it is a member.
39
40 If the element is not a member, do nothing.
41 """
42 pass
43
44 def intersection(self, *args, **kwargs): # real signature unknown
45 """
46 Return the intersection of two sets as a new set.
47
48 (i.e. all elements that are in both sets.)
49 """
50 pass
51
52 def intersection_update(self, *args, **kwargs): # real signature unknown
53 """ Update a set with the intersection of itself and another. """
54 pass
55
56 def isdisjoint(self, *args, **kwargs): # real signature unknown
57 """ Return True if two sets have a null intersection. """
58 pass
59
60 def issubset(self, *args, **kwargs): # real signature unknown
61 """ Report whether another set contains this set. """
62 pass
63
64 def issuperset(self, *args, **kwargs): # real signature unknown
65 """ Report whether this set contains another set. """
66 pass
67
68 def pop(self, *args, **kwargs): # real signature unknown
69 """
70 Remove and return an arbitrary set element.
71 Raises KeyError if the set is empty.
72 """
73 pass
74
75 def remove(self, *args, **kwargs): # real signature unknown
76 """
77 Remove an element from a set; it must be a member.
78
79 If the element is not a member, raise a KeyError.
80 """
81 pass
82
83 def symmetric_difference(self, *args, **kwargs): # real signature unknown
84 """
85 Return the symmetric difference of two sets as a new set.
86
87 (i.e. all elements that are in exactly one of the sets.)
88 """
89 pass
90
91 def symmetric_difference_update(self, *args, **kwargs): # real signature unknown
92 """ Update a set with the symmetric difference of itself and another. """
93 pass
94
95 def union(self, *args, **kwargs): # real signature unknown
96 """
97 Return the union of sets as a new set.
98
99 (i.e. all elements that are in either set.)
100 """
101 pass
102
103 def update(self, *args, **kwargs): # real signature unknown
104 """ Update a set with the union of itself and others. """
105 pass
106
107 def __and__(self, *args, **kwargs): # real signature unknown
108 """ Return self&value. """
109 pass
110
111 def __contains__(self, y): # real signature unknown; restored from __doc__
112 """ x.__contains__(y) <==> y in x. """
113 pass
114
115 def __eq__(self, *args, **kwargs): # real signature unknown
116 """ Return self==value. """
117 pass
118
119 def __getattribute__(self, *args, **kwargs): # real signature unknown
120 """ Return getattr(self, name). """
121 pass
122
123 def __ge__(self, *args, **kwargs): # real signature unknown
124 """ Return self>=value. """
125 pass
126
127 def __gt__(self, *args, **kwargs): # real signature unknown
128 """ Return self>value. """
129 pass
130
131 def __iand__(self, *args, **kwargs): # real signature unknown
132 """ Return self&=value. """
133 pass
134
135 def __init__(self, seq=()): # known special case of set.__init__
136 """
137 set() -> new empty set object
138 set(iterable) -> new set object
139
140 Build an unordered collection of unique elements.
141 # (copied from class doc)
142 """
143 pass
144
145 def __ior__(self, *args, **kwargs): # real signature unknown
146 """ Return self|=value. """
147 pass
148
149 def __isub__(self, *args, **kwargs): # real signature unknown
150 """ Return self-=value. """
151 pass
152
153 def __iter__(self, *args, **kwargs): # real signature unknown
154 """ Implement iter(self). """
155 pass
156
157 def __ixor__(self, *args, **kwargs): # real signature unknown
158 """ Return self^=value. """
159 pass
160
161 def __len__(self, *args, **kwargs): # real signature unknown
162 """ Return len(self). """
163 pass
164
165 def __le__(self, *args, **kwargs): # real signature unknown
166 """ Return self<=value. """
167 pass
168
169 def __lt__(self, *args, **kwargs): # real signature unknown
170 """ Return self<value. """
171 pass
172
173 @staticmethod # known case of __new__
174 def __new__(*args, **kwargs): # real signature unknown
175 """ Create and return a new object. See help(type) for accurate signature. """
176 pass
177
178 def __ne__(self, *args, **kwargs): # real signature unknown
179 """ Return self!=value. """
180 pass
181
182 def __or__(self, *args, **kwargs): # real signature unknown
183 """ Return self|value. """
184 pass
185
186 def __rand__(self, *args, **kwargs): # real signature unknown
187 """ Return value&self. """
188 pass
189
190 def __reduce__(self, *args, **kwargs): # real signature unknown
191 """ Return state information for pickling. """
192 pass
193
194 def __repr__(self, *args, **kwargs): # real signature unknown
195 """ Return repr(self). """
196 pass
197
198 def __ror__(self, *args, **kwargs): # real signature unknown
199 """ Return value|self. """
200 pass
201
202 def __rsub__(self, *args, **kwargs): # real signature unknown
203 """ Return value-self. """
204 pass
205
206 def __rxor__(self, *args, **kwargs): # real signature unknown
207 """ Return value^self. """
208 pass
209
210 def __sizeof__(self): # real signature unknown; restored from __doc__
211 """ S.__sizeof__() -> size of S in memory, in bytes """
212 pass
213
214 def __sub__(self, *args, **kwargs): # real signature unknown
215 """ Return self-value. """
216 pass
217
218 def __xor__(self, *args, **kwargs): # real signature unknown
219 """ Return self^value. """
220 pass
221
222 __hash__ = None
set
set功能属性虽多，但平时常用的也就那么几个。
常用属性
1. 添加元素
在集合中添加元素，可以使用add()方法，并且不生成一个新的集合。
#添加元素：add()
>>>s = {1,2,3}
>>>s.add(4)
>>>s
{1,2,3,4}
>>>s.add('g')
>>>s
{1,2,3,4,'g'}
>>>s.add(4)
>>>s
{1,2,3,4,'g'}
add()方法可以向set中添加元素，可以重复添加，但不会有效果。
2. 删除元素
set中利用remove()方法可以删除集合中的元素。
#删除元素
>>>s
{1,2,3,4,'g'}
>>>s.remove('g')
>>>s
{1,2,3,4}
3. 清空元素
clear()方法可以清空set中的元素。
#清空元素
>>>a = {1,2,3,4}
>>>b = a.clear()
>>>print(a,type(a))
set() <class 'set'>
>>>print(b,type(b))
None <class 'NoneType'>
4. 复制元素
copy()方法只能浅拷贝set中的元素，并生成一个新的集合。
#浅拷贝：copy()
>>>a = {1,(9,2),3}
>>>b = a.copy()
>>>print(a,id(a))
{(9, 2), 1, 3} 2097937619880
>>>print(b,id(b))
{(9, 2), 1, 3} 2097937620776 #赋值
>>>s = {1,2,3,4}
>>>d = s
>>>print(s,id(s))
{1, 2, 3, 4} 2097937785128
>>>print(d,id(d))
{1, 2, 3, 4} 2097937785128
5. pop()
pop()方法用于从set中随机取一个元素。记住，是随机的~~~
#pop()方法
>>>s = {1,2,3,4,5,'g','s'}
>>>s.pop()
'g'
>>>s.pop()
3
6. set集合操作
set与数学中的集合类似，是无序的和无重复元素的集合。因此，在Python中，set可以进行交集、并集、补集等操作。
| 数学符号 | Python符号 | 含义 |
| - 或\ | - | 差集，相对补集 |
| ∩ | & | 交集 |
| ∪ | | | 并集 |
| ≠ | != | 不等于 |
| ＝ | == | 等于 |
| ∈ | in | 是成员关系 |
| ∉ | not in | 非成员关系 |
#set集合操作
>>>s = {1,2,3,4}
>>>d = {2.3.5.6}
>>>s & d
{2.3}
>>>s | d
{1,2,3,4,5,6}
>>>s - d
{1,4}
>>>d - s
{5,6}
set和dict的唯一区别仅在于没有存储对应的value，但是，set的原理和dict一样，所以，同样不可以放入可变对象，因为无法判断两个可变对象是否相等，也就无法保证set内部“不会有重复元素”。因此，最常用的key是字符串。
“思想者”
set中存储着key，集合中不能放入可变的对象。之前的文章也说过：tuple是不可变的，而list是可变的。因此，set中是可以存储tuple的。这是真的吗？
时间是检验真理的唯一标准。下面请看示例代码：
#tuple可以作为集合中的元素
>>>s = {(1,),(1,2,3),1,2,'g'}
>>>s
{(1,),(1,2,3),1,2,'g'} #tuple也有失灵的时候
>>>t = (1,2,[1,2,3],4)
>>>type(t)
<class 'tuple'>
>>>d = {1,2,(1,2,[1,2,3],4)}
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
为什么会有错误呢？我也不清楚哎~~~这里面的道道很深，请读者细细体会。
set是一种数据结构。如果要详细的介绍set，我应该可以去出书了。这篇随笔只是起到入门的效果。
正所谓“师傅”领进门，修行靠大家嘛！