Python 避免使用元类继承生成的类属性

我在考虑自动将子类添加到父类中，以便使用元类进行“链接”。然而，从父类继承这些属性会把事情搞砸。有没有好办法避免这种情况

class MetaError(type):
    def __init__(cls, name, bases, attrs):
        for base in bases:
            setattr(base, name, cls)
        super(MetaError, cls).__init__(name, bases, attrs)

class BaseError(Exception, object):

    def __init__(self, message):
        super(BaseError, self).__init__(message)

class HttpError(BaseError):
    __metaclass__ = MetaError

class HttpBadRequest(HttpError):
    pass

class HttpNotFound(HttpError):
    pass

class FileNotFound(HttpNotFound):
    pass

class InvalidJson(HttpBadRequest):
    pass

http = HttpError

#  now I can do
raise http.HttpNotFound('Not found')
raise http.HttpNotFound.FileNotFound('File not found')
raise http.HttpBadRequest.InvalidJson('Invalid json')

#  unfortunately this also works
raise http.HttpBadRequest.HttpBadRequest('Bad request')
raise http.HttpBadRequest.HttpNotFound('Not found')

---

好吧，这比一开始看起来更棘手- 因为基本上您希望具有类继承关系，但不使用类继承上的常规属性查找路径- 否则，HTTPError作为BaseError的一个子类，总是在BaseError本身中存在所有属性-因此， 链

BaseError.HTTPError.HTTPError.HTTPError.HTTPError…

始终有效

幸运的是，Python确实提供了一种机制，可以将类注册为其他类的子类，而无需“物理”继承—也就是说，它被报告为子类，但其基类中没有父类或

\uuuuuuuuuuuuuuu

—因此，派生类上的属性查找（采用？）不会在“foster”中搜索属性家长

此机制通过“”或“abc”提供，通过其ABCMeta元类和“register”方法提供

现在，由于这个事实，你可能还想申报 您的类层次结构具有正常的继承语法-即， 能够写入

类HTTPError（BaseError）：

以指示新的 类派生自BaseError-您将获得实际的“物理”继承

因此，我们可以从ABCMeta类继承（而不是

type

）并编写

\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuu新方法，以便排除物理继承-
我们还使用
setattr
来包含您想要包含的代码，而且，我们还触发对
parentclass的所需调用。直接在元类上注册

（请注意，由于我们现在正在更改基类，因此需要修改
在元类的
\uuuuu new\uuuuu
方法中，而不是在
\uuuuu init\uuuuu
上：

from abc import ABCMeta

class MetaError(ABCMeta):
    def __new__(metacls, name, bases, attrs):

        new_bases = []
        base_iter = list(reversed(bases))
        seen = []
        register_this = None
        while base_iter:
            base = base_iter.pop(0)
            if base in seen:
                continue
            seen.append(base)
            if isinstance(base, MetaError):
                register_this = base
                base_iter = list(reversed(base.__mro__))  + base_iter
            else:
                new_bases.insert(0, base)
        cls = super(MetaError, metacls).__new__(metacls, name, tuple(new_bases), attrs)
        if register_this:
            setattr(register_this, name, cls)
            register_this.register(cls)
        return cls

对于快速测试：

class BaseError(Exception):
    __metaclass__ = MetaError
class HTTPError(BaseError):
    pass
class HTTPBadRequest(HTTPError):
    pass

在交互模式下，检查其是否按预期工作：

In [38]: BaseError.HTTPError
Out[38]: __main__.HTTPError

In [39]: BaseError.HTTPError.HTTPError
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-39-5d5d03751646> in <module>()
----> 1 BaseError.HTTPError.HTTPError

AttributeError: type object 'HTTPError' has no attribute 'HTTPError'

In [40]: HTTPError.__mro__
Out[40]: (__main__.HTTPError, Exception, BaseException, object)

In [41]: issubclass(HTTPError, BaseError)
Out[41]: True

In [42]: issubclass(HTTPBadRequest, BaseError)
Out[42]: True

In [43]: BaseError.HTTPError.HTTPBadRequest
Out[43]: __main__.HTTPBadRequest

In [44]: BaseError.HTTPBadRequest
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-44-b40d65ca66c6> in <module>()
----> 1 BaseError.HTTPBadRequest

AttributeError: type object 'BaseError' has no attribute 'HTTPBadRequest'

注意：无需显式地从
异常
和
对象
继承-
异常
本身已经从
对象
继承。而且，最重要的是：无论您在做什么项目，尽可能将其移动到Python3.x而不是Python2。Python2已经计算了天数，并且Python 3中有很多新特性，您无法使用。（这个答案中的代码与Python 2/3兼容，但是对于
\uuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuu

m = {}
class MetaError(type):

    def __init__(cls, name, bases, attrs):
        for base in bases:
            m[(base, name)] = cls 
        super(MetaError, cls).__init__(name, bases, attrs)

    def __getattribute__(self, value):
        if (self, value) in m:
            return m[self, value]
        return type.__getattribute__(self, value)

class BaseError(Exception):
    __metaclass__ = MetaError

class HttpError(BaseError):
    pass

class HttpBadRequest(HttpError):
    pass

class HttpNotFound(HttpError):
    pass

class FileNotFound(HttpNotFound):
    pass

class InvalidJson(HttpBadRequest):
    pass

谢谢你的回答。我很可能是最好的选择（或者，基于全球地图的解决方案也会起作用-作为答案发布）
m = {}
class MetaError(type):

    def __init__(cls, name, bases, attrs):
        for base in bases:
            m[(base, name)] = cls 
        super(MetaError, cls).__init__(name, bases, attrs)

    def __getattribute__(self, value):
        if (self, value) in m:
            return m[self, value]
        return type.__getattribute__(self, value)

class BaseError(Exception):
    __metaclass__ = MetaError

class HttpError(BaseError):
    pass

class HttpBadRequest(HttpError):
    pass

class HttpNotFound(HttpError):
    pass

class FileNotFound(HttpNotFound):
    pass

class InvalidJson(HttpBadRequest):
    pass