Python类继承和__dict__查找 [英] Python class inheritance and __dict__ lookup

问题描述

假设我定义了A类:

>>> class A:
...     a = 1
...     class SubA:
...         sub_a = { 'a': 1, 'b': 1}

然后我定义从A继承的类B:

Then I define class B that inherits from A:

>>> class B(A):
...     pass

现在，检查A的 __ dict __ 和B的 __ dict __ :

Now, check __dict__ of A and __dict__ of B:

>>> A.__dict__
{'a': 1, '__module__': '__builtin__', '__doc__': None, 'SubA': <class __builtin_ _.SubA at 0x02CAA3E8>}
>>> B.__dict__
{'__module__': '__builtin__', '__doc__': None}

以某种方式， B .__ dict __ 不包含'a'或'SubA'. 现在，如果这样做:

Somehow, B.__dict__ contains neither 'a' nor 'SubA'. Now if we do:

>>> A.a
1
>>> B.a
1

>>> A.SubA
<class __builtin__.SubA at 0x02CAA3E8>
>>> B.SubA
<class __builtin__.SubA at 0x02CAA3E8>

第一个问题:为什么 B .__ dict __ 不包含'a'和'SubA'? 第二个问题:为什么'a'和'SubA'都不在 B 的<强> __ dict __ ?

First question: why B.__dict__ does not contain 'a' and 'SubA'? Second question: Why B.a and B.SubA give the expected results, although neither 'a' nor 'SubA' is in B's __dict__?

谢谢！

推荐答案

@bgporter为行为提供了很好的解释，我只介绍一下为什么:

@bgporter has given a good explanation of the behaviour, I'll just go into why a little:

如果您的类变量在B.__dict__中，它将如何运行?每个子类都有自己的a值，与A.a的值无关-这不是您所期望的.一个类变量应该只存在一次-在该类中.

If your class variable was in B.__dict__, how would it function? Each subclass would have it's own value for a, independent of the value for A.a - this is not what you would expect. A class variable should exist once - in that class.

相反，Python对类进行查找，如果不存在，则查找其基类-请注意，这意味着可以在子类中隐藏类变量.

Instead, Python does a lookup on the class and if it doesn't exist, then looks up to it's base classes - note that means it is possible to shadow a class variable in a subclass.

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