泛型元类来跟踪子类? [英] Generic metaclass to keep track of subclasses?

问题描述

我正在尝试编写一个通用的元类来跟踪子类

I'm trying to write a generic metaclass to track subclasses

由于我希望它是通用的，所以我不想在此元类中对任何类名进行硬编码，因此我想出了一个生成适当元类的函数，例如:

Since I want this to be generic, I didn't want to hardcode any class name within this metaclass, therefore I came up with a function that generates the proper metaclass, something like:

def make_subtracker(root):
    class SubclassTracker(type):
        def __init__(cls, name, bases, dct):
            print('registering %s' % (name,))
            root._registry.append(cls)
            super(SubclassTracker, cls).__init__(name, bases, dct)
    return SubclassTracker

这样，我可以调用它来为特定的 root 类生成一个元类，

This way I could invoke it to generate a metaclass for a specific root class with:

__metaclass__ = make_subtracker(Root)

这是我遇到问题的地方.我不能这样做:

Here is where I bump into a problem. I cannot do this:

class Root(object):
   _registry = []
   __metaclass__ = make_subtracker(Root)

...因为使用make_subtracker(Root)时尚未定义Root.我稍后尝试添加__metaclass__属性，以便至少可以将其应用于子类:

...because Root is not defined yet when I use make_subtracker(Root). I tried adding the __metaclass__ attribute later, so that at least it can be applied in subclasses:

class Root(object):
   _registry = []

Root.__metaclass__ = make_subtracker(Root)

...但这不起作用.读取类定义时，__metaclass__具有特殊处理，如自定义类创建.

...but this doesn't work. __metaclass__ has a special processing when the class definition is read, as defined in Customizing class creation.

我正在为此寻求建议(或者在运行时以将其应用于子类的方式更改类的元类，或者其他任何替代方法).

I'm looking for suggestions in order to do this (either change a class' metaclass at runtime in a way that it is applied to its subclasses, or any other alternative).

推荐答案

Python针对新类自动执行此操作，如本在这里.

Python does this automatically for new-style classes, as mentioned in this answer to the similar queston How to find all the subclasses of a class given its name? here.

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