如何创建一个简单的元类？

问题描述

我一直在做Python一段时间，我总是有点理解元类的意义，但我从来没有需要一个。
现在我认为我的问题的最佳解决方案是一个元类（如果有更好的方法，请更正我）。

I've been doing Python for some time now, and I've always somewhat understood the meaning of metaclasses, but I've never needed one.Now I think the best solution for my problem is a metaclass (correct me if there's a better way).

我想创建的是一个系统自动向每个类别添加类变量 n 和列表 instances 。这里有一个简单的例子：

What I'm trying to create is a system which automatically adds a class variable n and a list instances to each class of mine. Here's a simplified example of one class:

class Foo:
    n = 0
    instances = []

    def __init__(self):
        self.index = Foo.n
        Foo.n += 1
        Foo.instances.append(self)

这个结构应该实现我的7或8类，我想一个元类可能会帮助我。
我知道我可以使用 Foo .__ metaclass__ = MyMetaclass 属性来使用元类，但是如何 / strong>元类？

This structure should be implemented for 7 or 8 classes of mine, and I was thinking that a metaclass might help me here.I know I can use the Foo.__metaclass__ = MyMetaclass attribute to use the metaclass, but how do I create the metaclass?

推荐答案

实际上，使用基类会更好：

Actually, using a base class would work out better here:

class InstancesList(object): 
    def __new__(cls, *args, **kw):
        if not hasattr(cls, 'instances'):
            cls.instances = []
        return super(InstancesList, cls).__new__(cls, *args, **kw)

    def __init__(self):
        self.index = len(type(self).instances)
        type(self).instances.append(self)

class Foo(InstancesList):
    def __init__(self, arg1, arg2):
        super(Foo, self).__init__()
        # Foo-specific initialization

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