April 2023
Intermediate to advanced
769 pages
25h 16m
Chinese
Content preview from 流畅的Python（第2版）
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74
｜
第
3
章
>>> chain['c'] = -1
>>> d1
{'a': 1, 'b': 3, 'c': -1}
>>> d2
{'a': 2, 'b': 4, 'c': 6}
ChainMap
可用于实现支持嵌套作用域的语言解释器，按嵌套层级从内到外，一个映射表示
一个作用域上下文。
collections
文档中的“
ChainMap objects
”一节举了几个
ChainMap
用
法示例，其中一个就是模仿
Python
查找变量的基本规则，如下所示。
import builtins
pylookup = ChainMap(locals(), globals(), vars(builtins))
第
18
章中的示例
18-14
使用一个
ChainMap
子类为
Scheme
编程语言的子集实现解释器。
3.6.3
collections.Counter
这是一种对键计数的映射。更新现有的键，计数随之增加。可用于统计可哈希对象的实例
数量，或者作为多重集（
multis
et
，本节后文讨论）使用。
Counter
实现了组合计数的
+
和
-
运算符，以及其他一些有用的方法，例如
most_common([n])
。该方法返回一个有序元组
列表，对应前
n
个计数值最大的项及其数量。下面使用
Counter
统计词中的字母数量。
>>> ct = collections.Counter('abracadabra')
>>> ct
Counter({'a': 5, 'b': 2, 'r': 2, 'c': 1, 'd': 1})
>>> ...
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