一个自己的类子类。为什么禁止相互子类化？ [英] A class subclass of itself. Why mutual subclassing is forbidden?

问题描述

复杂的问题我假设，但研究OWL打开了一个新的视角，生活，宇宙和一切。

Complex question I assume, but studying OWL opened a new perspective to live, the universe and everything. I'm going philosophical here.

我想实现一个C类，它是B的子类，它又是C的子类。只是为了有趣，你知道。 ..

I am trying to achieve a class C which is subclass of B which in turn is subclass of C. Just for fun, you know...

这里是

>>> class A(object): pass
... 
>>> class B(A): pass
... 
>>> class C(B): pass
... 
>>> B.__bases__
(<class '__main__.A'>,)
>>> B.__bases__ = (C,)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: a __bases__ item causes an inheritance cycle
>>> 

很清楚，python是聪明的，禁止这。然而，在OWL中，可以将两个类定义为相互子类。问题是：为什么在OWL（这不是一种编程语言）中允许使用并且不允许在编程语言中使用？为什么这个问题是令人费解的？

clearly, python is smart and forbids this. However, in OWL it is possible to define two classes to be mutual subclasses. The question is: what is the mind boggling explanation why this is allowed in OWL (which is not a programming language) and disallowed in programming languages ?

推荐答案

Python不允许它，因为没有明智的方法来做到这一点。你可以发明关于如何处理这种情况的任意规则（也许有一些语言做），但是由于没有实际的收获，Python拒绝猜测。由于多种原因，类需要具有稳定，可预测的方法解析顺序，因此不允许出现奇怪，不可预测或令人惊讶的MRO。

Python doesn't allow it because there is no sensible way to do it. You could invent arbitrary rules about how to handle such a case (and perhaps some languages do), but since there is no actual gain in doing so, Python refuses to guess. Classes are required to have a stable, predictable method resolution order for a number of reasons, and so weird, unpredictable or surprising MROs are not allowed.

em>是在Python中的特殊情况： type 和 object 。 对象是类型的实例，类型是 object 的子类。当然， type 也是 一个类型的实例（因为它是 object ）。这可能是为什么OWL允许它：如果你想让一切都是一个对象，并且所有对象都有一个类，你需要以一个奇异的方式启动一个类/元类层次。

That said, there is a special case in Python: type and object. object is an instance of type, and type is a subclass of object. And of course, type is also an instance of type (since it's a subclass of object). This might be why OWL allows it: you need to start a class/metaclass hierarchy in some singularity, if you want everything to be an object and all objects to have a class.

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