Python中UserDict类的优点 [英] Advantages of UserDict class in Python

问题描述

使用 UserDict 类有什么优势?

我的意思是，我真正得到的是，而不是

I mean, what I really get if instead of

class MyClass(object):
    def __init__(self):
        self.a = 0
        self.b = 0
...
m = MyClass()
m.a = 5
m.b = 7

我将写以下内容:

class MyClass(UserDict):
    def __init__(self):
        UserDict.__init__(self)
        self["a"] = 0
        self["b"] = 0
...
m = MyClass()
m["a"] = 5
m["b"] = 7

编辑:如果我理解正确，那么在两种情况下都可以在运行时中向对象添加新字段吗?

Edit: If I understand right I can add new fields to an object in a runtime in both cases?

m.c = "Cool"

和

m["c"] = "Cool"

推荐答案

UserDict.UserDict 自Python 2.2起没有实质性的附加值，因为正如@gs所提到的，您现在可以直接将dict子类化-它仅存在于与Python 2.1及更低版本的向后兼容中，而内置类型无法实现.子类.尽管如此，它还是作为collections模块中的适当位置). UserDict"rel =" noreferrer>文档现在提到，

UserDict.UserDict has no substantial added value since Python 2.2, since, as @gs mention, you can now subclass dict directly -- it exists only for backwards compatibility with Python 2.1 and earlier, when builtin types could not be subclasses. Still, it was kept in Python 3 (now in its proper place in the collections module) since, as the docs now mention,

对此类的需求已经 被以下能力部分取代 直接从dict继承子类；然而， 这节课可以更容易地使用 因为基础字典是 可作为属性访问.

The need for this class has been partially supplanted by the ability to subclass directly from dict; however, this class can be easier to work with because the underlying dictionary is accessible as an attribute.

UserDict.DictMixin，在Python 2中非常方便-正如文档所述，

UserDict.DictMixin, in Python 2, is quite handy -- as the docs say,

该模块定义了一个mixin DictMixin， 定义所有的字典方法 已经有最低限度的课程 映射接口.这个很大 简化需要编写的类 可以代替字典 (例如货架模块).

The module defines a mixin, DictMixin, defining all dictionary methods for classes that already have a minimum mapping interface. This greatly simplifies writing classes that need to be substitutable for dictionaries (such as the shelve module).

将其子类化，定义一些基本方法(至少__getitem__，这对于不具有获取键或迭代能力的只读映射就足够了；如果需要这些功能，还可以keys；可能是__setitem__ ，并且您具有R/W映射，但无法删除项目；添加__delitem__以获得完整功能，并可能出于性能原因而覆盖其他方法)，并获得dict丰富API的完整实现(update，get等). 模板方法设计模式的一个很好的例子.

You subclass it, define some fundamental methods (at least __getitem__, which is sufficient for a read-only mapping without the ability to get keys or iterate; also keys if you need those abilities; possibly __setitem__, and you have a R/W mapping without the ability of removing items; add __delitem__ for full capability, and possibly override other methods for reasons of performance), and get a full-fledged implementation of dict's rich API (update, get, and so on). A great example of the Template Method design pattern.

在Python 3中，DictMixin不见了；您可以通过依赖collections.MutableMapping来获得几乎相同的功能(或者对于R/O映射，只需collections.Mapping即可).它虽然没有那么方便，但还是有点优雅(请参阅此问题，该文档以"won"结束了修复"；简短的讨论值得一读).

In Python 3, DictMixin is gone; you can get almost the same functionality by relying on collections.MutableMapping instead (or just collections.Mapping for R/O mappings). It's a bit more elegant, though not QUITE as handy (see this issue, which was closed with "won't fix"; the short discussion is worth reading).

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