为什么__dict__类是一个映射代理？ [英] Why is a class __dict__ a mappingproxy?

问题描述

我想知道为什么类 __ dict __ 是 mappingproxy ，但是是实例 __ dict __ 只是普通的 dict

I wonder why a class __dict__ is a mappingproxy, but an instance __dict__ is just a plain dict

>>> class A:
...     pass

>>> a = A()
>>> type(a.__dict__)
<class 'dict'>
>>> type(A.__dict__)
<class 'mappingproxy'>

推荐答案

这有助于解释程序确保class-级别属性和方法只能是字符串。

This helps the interpreter assure that the keys for class-level attributes and methods can only be strings.

在其他地方，Python是一种成人同意语言，这意味着对象的指示由用户公开和可变。但是，对于类的类级属性和方法，如果我们可以保证键是字符串，则可以在类级简化和加快用于属性和方法查找的常见案例代码。尤其是，通过假定类dict键为字符串，可以简化和加快用于新型类的__mro__搜索逻辑。

Elsewhere, Python is a "consenting adults language", meaning that dicts for objects are exposed and mutable by the user. However, in the case of class-level attributes and methods for classes, if we can guarantee that the keys are strings, we can simplify and speed-up the common case code for attribute and method lookup at the class-level. In particular, the __mro__ search logic for new-style classes is simplified and sped-up by assuming the class dict keys are strings.

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