python3.x 基础三：set集合

集合，set()，记住：

1个特点：去重，把列表变成集合，达到自动去重操作，无序

5个关系：测试两个列表的交差并子反向差集

方法：

•  |  add(...) 常用，已存在元素去重不生效
•  |      Add an element to a set.
•  |     This has no effect if the element is already present.

  >>> list1=[3,2,1,1,2,3,4,5]
  >>> set(list1)
  {1, 2, 3, 4, 5}
  >>> list2=[3,4,5,6,7,8]
  >>> set(list1).add(2)
  >>> set(list1).add(6)
  >>> print(set(list1).add(2))
  None
  >>> print(set(list1).add(6))
  None
  >>> set1=set(list1)
  >>> set2=set(list2)
  >>> set1,set2
  ({1, 2, 3, 4, 5}, {3, 4, 5, 6, 7, 8})
  >>> set1.add(3)
  >>> print(set1.add(3)
  ... )
  None
  >>> print(set1.add(7))
  None
  >>> set1.add('aaa')
  >>> set1
  {1, 2, 3, 4, 5, 7, 'aaa'}
  >>> id(set1)
  140138768484616
  >>> set1.add('aaaa')
  >>> id(set1)
  140138768484616
  >>> set1
  {1, 2, 3, 4, 5, 7, 'aaa', 'aaaa'}
  >>> set1.add('7')
  >>> set1
  {1, 2, 3, 4, 5, 7, 'aaa', 'aaaa', '7'}
  >>> set1.add(7)
  >>> set1
  {1, 2, 3, 4, 5, 7, 'aaa', 'aaaa', '7'}

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