Python中集合的内建函数和内建方法学习教程
集合内建函数和内建方法
（1）标准类型函数
len()：把集合作为参数传递给内建函数 len()，返回集合的基数（或元素的个数）。
（2）集合类型工厂函数
set()和 frozenset()工厂函数分别用来生成可变和不可变的集合。如果不提供任何参数，默认会生成空集合。如果提供一个参数，则该参数必须是可迭代的，即一个序列或迭代器或支持迭代的一个对象，例如一个文件或一个字典。
（3）方法(所有的集合方法)
s.issubset(t) 如果s是t的子集,则返回True,否则返回False
s.issuperset(t) 如果t是s的超集,则返回True,否则返回False
s.union(t) 返回一个新集合,该集合是s和t的并集
s.intersection(t) 返回一个新集合,该集合是s和t的交集
s.difference(t) 返回一个新集合,该集合是 s 的成员,但不是 t 的成员
s.symmetric_difference(t) 返回一个新集合,该集合是s或t的成员,但不是s和t共有的成员
s.copy() 返回一个新集合,它是集合s的浅复制
内建方法copy() 没有等价的操作符。和同名的字典方法一样，copy()方法比用像set()、frozenset()或dict()这样的工厂方法复制对象的副本要快。
（4）方法(仅适用于可变集合)
可变集合类型的方法：
演示实例：
一、集合类型方法
>>> s = set('cheeseshop')
>>> t = set('bookshop')
>>> s
set(['c', 'e', 'h', 'o', 'p', 's'])
>>> t
set(['b', 'h', 'k', 'o', 'p', 's'])
>>> s.issubset(t)
False
>>> s.issuperset(t)
False
>>> s.union(t)
set(['c', 'b', 'e', 'h', 'k', 'o', 'p', 's'])
>>> s.intersection(t)
set(['h', 's', 'o', 'p'])
>>> s.difference(t)
set(['c', 'e'])
>>> s.symmetric_difference(t)
set(['b', 'e', 'k', 'c'])
>>> s.copy()
set(['p', 'c', 'e', 'h', 's', 'o'])
二、可变集合类型的方法
1、s.update(t)——用t中的元素修改s，即s现在包含s或t的成员。
>>> s.update(t) >>> s set(['c', 'b', 'e', 'h', 'k', 'o', 'p', 's'])
2、s.intersection_update(t)——s中的成员是共同属于s和t中的元素。
>>> s = set('cheeseshop')
>>> t = set('bookshop')
>>> s.intersection_update(t)
>>> s
set(['h', 's', 'o', 'p'])
3、s.difference_update(t)——s中的成员是属于s但不包含在t中的元素。
>>> s = set('cheeseshop')
>>> t = set('bookshop')
>>> s.difference_update(t)
>>> s
set(['c', 'e'])
4、s.symmetric_difference_update(t)——s中的成员更新为那些包含在s或t中，但不是s和t共有的元素。
>>> s = set('cheeseshop')
>>> t = set('bookshop')
>>> s.symmetric_difference_update(t)
>>> s
set(['c', 'b', 'e', 'k'])
5、s.add(obj)——在集合s中添加对象obj。
>>> s.add('o')
>>> s
set(['c', 'b', 'e', 'k', 'o'])
6、s.remove(obj)——从集合s中删除对象obj，如果obj不是集合s中的元素（obj not in s），将引发KeyError。
<p>>>> s.remove('b')
>>> s
set(['c', 'e', 'k', 'o'])
>>> s.remove('a')</p><p>Traceback (most recent call last):
File "<pyshell#53>", line 1, in <module>
s.remove('a')
KeyError: 'a'
</p>
7、s.discard(obj)——如果obj是集合s中的元素，从集合s中删除对象obj。
>>> s.discard('a')
>>> s
set(['c', 'e', 'k', 'o'])
>>> s.discard('e')
>>> s
set(['c', 'k', 'o'])
8、s.pop()——删除集合是中的任意一个对象，并返回它。
>>> s.pop() 'c' >>> s set(['k', 'o'])
9、s.clear()——删除集合s中的所有元素。
>>> s.clear() >>> s set([])
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