如何在Python中初始化类(而不是实例)？

Question

我想只在加载一个类(而不是每个对象!)时合并当前和继承类的约束.

class Domain(Validatable):

    constraints = {...}

为此,我定义了一个_initialize_class_not_instance应该为每个类调用一次的方法:

class Validatable:

    @classmethod
    def _initialize_class_not_instance(cls):
        # merge constraints from derived class and base classes
        pass

    __class__._initialize_class_not_instance() # doesn't work
    # Validatable._merge_constraints() # doesn't work too

问题是__class__在这种情况下不存在,Validatable也没有定义.但是我想避免,我的API的用户必须明确地调用initialize方法或者必须使用额外的类装饰器.

任何想法如何初始化类？

Answer 1

使用元类.

class MetaClass(type):
  def __init__(cls, name, bases, d):
    type.__init__(cls, name, bases, d)
    cls.foo = 42

class MyClass(object):
  __metaclass__ = MetaClass

print MyClass.foo

Answer 2

我最终得到了一个类装饰器并且没有继承。该装饰器合并当前类和继承类的约束并重写该__init__方法：

def validatable(cls):

    # merge constraints from derived class and base classes here ...

    original_init = cls.__init__
    def init_to_insert(self, *args, **keywords):
        self.errors = {}
        original_init(self, *args, **keywords)   
    cls.__init__ = init_to_insert
    return cls

这是我的解决方案的关键，因为装饰器必须修改类（合并约束和插入方法）和实例。