看python官方说明学python-set-集合_python from sets-优快云博客

本文介绍了Python中的集合数据类型，包括创建、基本使用、数学运算和集合操作等，通过示例演示了如何去除重复元素、快速进行成员测试及执行交集、并集、差集和对称差集等操作。

5.4. Sets

Python also includes a data type for sets. A set is an unordered collection with no duplicate elements. Basic uses include membership testing and eliminating duplicate entries. Set objects also support mathematical operations like union, intersection, difference, and symmetric difference.

Curly braces or the set() function can be used to create sets. Note: to create an empty set you have to use set(), not{}; the latter creates an empty dictionary, a data structure that we discuss in the next section. python3.3里面这样用。

Here is a brief demonstration:

   >>> 
   >>> basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
>>> print(basket)                      # show that duplicates have been removed
{'orange', 'banana', 'pear', 'apple'}
>>> 'orange' in basket                 # fast membership testing
True
>>> 'crabgrass' in basket
False

>>> # Demonstrate set operations on unique letters from two words
...
>>> a = set('abracadabra')
>>> b = set('alacazam')
>>> a                                  # unique letters in a
{'a', 'r', 'b', 'c', 'd'}           #这里要说明一下python3中集合set显示直接用字典dict的表示方法了，在python2中集合a 的值是set(['a','r','b','c','d'])。相比起来python3好看了许多。
>>> a - b                              # letters in a but not in b
{'r', 'd', 'b'}
>>> a | b                              # letters in either a or b
{'a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'}
>>> a & b                              # letters in both a and b
{'a', 'c'}
>>> a ^ b                              # letters in a or b but not both
{'r', 'd', 'b', 'm', 'z', 'l'}
 
  

Similarly to list comprehensions, set comprehensions are also supported:

>>>>>> a = {x for x in 'abracadabra' if x not in 'abc'}
>>> a
{'r', 'd'}                    #这个效果跟 上面 例子中两个集合相减是一个效果a - b