定义集合：用{}
再次总结一下，元组tuple是(),list是[],集合是{}
>>> type({1,2,3,4,5})
<class 'set'>

如何定义空集？
 type({})
<class 'dict'>
这肯定不行
得这样
>>> type(set())
<class 'set'>

与list相比 集合不支持下标操作，那更不可能支持切片操作
>>> {1,2,3}[2]
<stdin>:1: SyntaxWarning: 'set' object is not subscriptable; perhaps you missed a comma?
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'set' object is not subscriptable
>>>
>>> {1,2,3}[0:2]
<stdin>:1: SyntaxWarning: 'set' object is not subscriptable; perhaps you missed a comma?
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'set' object is not subscriptable

集合也支持len(),计算长度
集合是不包含重复的元素的

>>> len({1,2,3})
3
>>> {1,2,3}
{1, 2, 3}
>>> {1,2,3,3}
{1, 2, 3}
>>> {1,2,3,4}
{1, 2, 3, 4}

跟list tuple 一样，支持这种操作
>>> 1 in {1,2,3}
True
>>> 1 in [1,2,3]
True
>>> 1 in (1,2,3)
True

支持差集合运算、交集运算、并集运算
>>> {1,2,3}-{1,2}
{3}
>>> {1,2,3}&{1,2}
{1, 2}
>>> {1,2,3}| {3,4}
{1, 2, 3, 4}