为什么类 __dict__ 是映射代理? [英] Why is a class __dict__ a mappingproxy?

问题描述

我想知道为什么一个类 __dict__ 是一个 mappingproxy，而一个实例 __dict__ 只是一个普通的 dict

<预><代码>>>>A类:... 经过>>>a = A()>>>类型(a.__dict__)<类'dict'>>>>类型(A.__dict__)<类'映射代理'>

解决方案

这有助于解释器确保类级属性和方法的键只能是字符串.

在其他地方，Python 是一种成人同意的语言"，这意味着对象的 dicts 是公开的并且可由用户更改.但是，对于类的类级别属性和方法的情况，如果我们能够保证键是字符串，我们就可以简化和加快类级别属性和方法查找的常见案例代码.特别是，新式类的 __mro__ 搜索逻辑通过假设类 dict 键是字符串来简化和加速.

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'>

解决方案

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

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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