派生时自动类装饰(或验证)

Question

我有一个基类,我从中派生出多个子类.
每个子类定义类常量,我希望对它们强制执行某些限制.例如:

class Base(object):
    # define these in your sub-class, and make sure (NOM % DENOM == 0)
    NOMINATOR = None
    DENOMINATOR = None

class Subclass_good(Base):
    NOMINATOR = 6
    DENOMINATOR = 3

class Subclass_bad(Base):
    NOMINATOR = 7
    DENOMINATOR = 5

我希望能够执行规则(NOM%DENOM == 0).
我目前使用类装饰器执行此操作:

def nom_denom_validator(cls):
    assert(cls.NOMINATOR % cls.DENOMINATOR == 0)
    return cls

# and decorate each subclass, e.g.:
@nom_denom_validator
class Subclass_another(Base):
    NOMINATOR = 9
    DENOMINATOR = 12

但我不喜欢我需要装饰每个子类的事实(我有很多).我很感兴趣是否可以通过直接对Base类进行一些操作来完成.

有什么建议？

Answer 1

好的,好笑 我想了一段时间,但只是在发布问题后 - 特别是在选择标签时,并在那里添加"元类" - 我才意识到自己可能有一个答案.
因此,提交审查和未来的知识,这里是:

class Base_Metaclass(type):
    def __new__(meta, classname, bases, class_dict):
        new_type = type.__new__(meta, classname, bases, class_dict)
        if not (new_type.NOMINATOR % new_type.DENOMINATOR) == 0:
            raise Exception("Invalid subclass created - validation failed")
        return new_type

# have Base and all its descendants be enforced:
class Base(object):
    __metaclass__ = Base_Metaclass
    # I must pass the validation myself, no None's anymore...
    NOMINATOR = 1
    DENOMINATOR = 1

现在所有的孩子都应该自动执行.