元类如何访问子类上定义的属性？

Posted 2023-02-23

【中文标题】元类如何访问子类上定义的属性？

【英文标题】：How can a metaclass access attributes defined on a child class?

【发布时间】：2021-10-21 15:58:45

【问题描述】：

我正在开发一个类生成器。我希望能够创建许多BasePermission 的子代，这些子代可以使用required_scopes 属性进行自定义。这是我目前所拥有的。

from rest_framework.permissions import BasePermission


class ScopedPermissionMeta(type(BasePermission)):

    def __init__(self, name, bases, attrs):
        try:
            required_scopes = attrs['required_scopes']
        except KeyError:
            raise TypeError(f'name must include required_scopes attribute.')

        required_scopes_list = ' '.join(required_scopes)
        attrs['message'] = f'Resource requires scope=required_scopes_list'

        def has_permission(self, request, _view):
            """Check required scopes against requested scopes."""
            try:
                requested_scopes = request.auth.claims['scope']
            except (AttributeError, KeyError):
                return False

            return all(scope in requested_scopes for scope in required_scopes)

        attrs['has_permission'] = has_permission


class ReadJwtPermission(BasePermission, metaclass=ScopedPermissionMeta):
    required_scopes = ['read:jwt']

但是，我不喜欢ReadJwtPermisson 类（以及更多的子类）必须指定一个元类。理想情况下，我希望将这些细节抽象掉。我希望能够做类似的事情：

class ScopedPermission(BasePermission, metaclass=ScopedPermissionMeta):
    pass


class ReadJwtPermission(ScopedPermission):
    required_scopes = ['read:jwt']

但在这种情况下，元类看到的是ScopedPermission，而不是required_scopes。有没有办法让元类看穿这种继承关系？

【参考方案1】：

但在这种情况下，元类看到的是 ScopedPermission 而没有 required_scopes。有没有办法让元类看穿 这种继承关系？

在创建ScopedPermission 类的那一刻，没有ReadJwtPermission 类。解释器无法预测未来一个类将继承 ScopedPermission 的子类，它具有 required_scopes 属性。但是你可以做一些不同的事情。

子类继承父类的元类。如果父类使用该元类，则每个子类都必须具有您想要的属性。我还使用__new__ 在类创建“之前”检查该属性。这是一个例子：

class Metaclass(type):
    def __new__(mcs, name, bases, attrs):

        # This condition skips Base class's requiement for having "required_scopes"
        # otherwise you should specify "required_scopes" for Base class as well.
        if name == 'Base':
            return super().__new__(mcs, name, bases, attrs)

        try:
            required_scopes = attrs['required_scopes']
        except KeyError:
            raise TypeError(f'name must include "required_scopes" attribute.')

        required_scopes_list = ' '.join(required_scopes)
        attrs['message'] = f'Resource requires scope=required_scopes_list'

        # setting "has_permission attribute here"
        attrs['has_permission'] = mcs.has_permission()

        return super().__new__(mcs, name, bases, attrs)

    # I just removed the implementation so that I can be able to run this class.
    @staticmethod
    def has_permission():
        pass


class Base(metaclass=Metaclass):
    pass

class A(Base):
    required_scopes = ['read:jwt']

print(A.message)

输出：

Resource requires scope=read:jwt

但现在有了：

class B(Base):
    pass

它引发错误...

Traceback (most recent call last):
  File "<--->", line 9, in __new__
    required_scopes = attrs['required_scopes']
KeyError: 'required_scopes'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<--->", line 34, in <module>
    class B(Base):
  File "<--->", line 11, in __new__
    raise TypeError(f'name must include "required_scopes" attribute.')
TypeError: B must include "required_scopes" attribute.

【讨论】：

我在你的代码中唯一改变的是class Base(BasePermission, metaclass=ScopedPermissionMeta):和class Metaclass(type(BasePermission)):，否则它会按预期工作，谢谢

@BradyDean 太好了，我只关注另一部分。