python的most_common()函数

我们知道python内建模块的collections有很多好用的操作。

比如：

from collections import Counter
#统计字符串
# top n问题
user_counter = Counter("abbafafpskaag")
print(user_counter.most_common(3)) #[('a', 5), ('b', 2), ('f', 2)]
print(user_counter['a']) # 5

python里面实现most_common:

 def most_common(self, n=None):
        '''List the n most common elements and their counts from the most
        common to the least.  If n is None, then list all element counts.

        >>> Counter('abcdeabcdabcaba').most_common(3)
        [('a', 5), ('b', 4), ('c', 3)]

        '''
        # Emulate Bag.sortedByCount from Smalltalk
        if n is None:
            return sorted(self.items(), key=_itemgetter(1), reverse=True)
        return _heapq.nlargest(n, self.items(), key=_itemgetter(1))

这里用到了个 _heapq 堆数据结构 也就是说它是堆来解决top n问题的，而不是遍历。

总结：most_common()函数用来实现Top n 功能.