如何在 Python 中初始化类（不是实例）？

Question

I want to merge constraints from the current and inherited classes only once a class is loaded (not per object.).

class Domain(Validatable):

    constraints = {...}

To do this I defined a method _initialize_class_not_instance that should be called once for each class:

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

The problem is that __class__ doesn't exist in this context and Validatable is not defined too. But I want to avoid, that the user of my API has to call the initialize method explicitely or has to use an additional class decorator.

Any ideas how to initialize the class?

Answer 1

Use a metaclass.

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

I ended up with a class decorator and without inheritance. This decorator merges the constraints of the current and inherited classes and overrides the __init__ method:

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

This is the key for my solution, since the decorator has to modify the class (merging constraints and inserting methods) and instances.