Python 还包含了一个数据类型 —— set （集合）。集合是一个无序不重复元素的集。基本功能包括关系测试和消除重复元素。集合对象还支持 union（联合），intersection（交），difference（差）和 sysmmetric difference（对称差集）等数学运算。

大括号或 set() 函数可以用来创建集合。注意：想要创建空集合，你必须使用 set() 而不是 {}。后者用于创建空字典，我们在下一节中介绍的一种数据结构。

以下是一个简单的演示:

>>> 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"}
>>> 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"}

类似 for lists，这里有一种集合推导式语法:

>>> a = {x for x in "abracadabra" if x not in "abc"}
>>> a
{"r", "d"}