April 2023
Intermediate to advanced
769 pages
25h 16m
Chinese
Content preview from 流畅的Python（第2版）
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64
｜
第
3
章
3.4
映射类型的标准
API
collections.abc
模块中的抽象基类
Mapping
和
MutableMapping
描述
dict
和类似类型的接
口，见图
3-1
。
这两个抽象基类的主要作用是确立映射对象的标准接口，并在需要广义上的映射对象时为
isinstance
提供测试标准。
>>> my_dict = {}
>>> isinstance(my_dict, abc.Mapping)
True
>>> isinstance(my_dict, abc.MutableMapping)
True
提示
使用
isinstance
测试是否满足抽象基类的接口要求，往往比检查一个函数的
参数是不是具体的
dict
类型要好，因为除了
dict
之外还有其他映射类型可
用。这个问题将在第
13
章详谈。
图 3-1：
MutableMapping
及其在
collections.abc
中的超类的简化 UML 类图
如果想自定义映射类型，扩展
collections.UserDict
或通过组合模式包装
dict
更简单，那
就不要定义这些抽象基类的子类。
collections.UserDict
类和标准库中的所有具体映射类
都在实现中封装了基本的
dict
，而
dict
又建立在哈希表之上。因此，这些类有一个共同
的限制，即键必须
可哈希
（值不要求可哈希，只有键需要）。
3.4.1
节会讲
一讲相关概念，
帮你回顾一下。
3.4.1
“
可哈希
”
指什么
“可哈希”在
Python
术语表中有定义，摘录（有部分改动）如下：
如果一个对象的哈希码在整个生命周期内永不改变（依托 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.