metaclass example with self registering descriptors

descriptors.py

class Field(object):
    def __init__(self, name):
        self._name = name

    def __get__(self, instance, owner):
        return instance._data[self._name]

    def __set__(self, instance, value):
        instance._data[self._name] = value


class IntegerField(Field):
    pass


class StringField(Field):
    pass


class ModelMeta(type):

    _registry = {}

    def __new__(cls, name, bases, attrs):
        for n, v in attrs.items():
            if isinstance(v, Field):
                if name not in cls._registry:
                    cls._registry[name] = []
                cls._registry[name].append(n)
        return super(ModelMeta, cls).__new__(cls, name, bases, attrs)


class Model(object):
    __metaclass__ = ModelMeta

    def __init__(self):
        self._data = {}

    def fields(self):
        return ModelMeta._registry[self.__class__.__name__]

    def __repr__(self):
        str = "<%s " % self.__class__.__name__
        fields = []
        for field in ModelMeta._registry[self.__class__.__name__]:
            fields.append("%s=%s" % (field, getattr(self, field)))
        str += ", ".join(fields)
        str += ">"
        return str


class Circle(Model):

    diameter = StringField('diameter')


class Square(Model):
    width = StringField('width')


if __name__ == '__main__':
    model = Circle()
    model.diameter = 1
    print model.diameter
    print model

    print model.fields()


    square = Square()
    square.width = 5
    print square
    print square.fields()