NullProxy and Borg objects

gistfile1.py

class NullProxy(object):
    """Pretend you have a model object with three attributes:

    class Person(object):
        name = String(40)
        age = Integer()
        email = Email()

    And have a sub-class adding extra attributes:

    class SpecialPerson(Person):
        role = "true believer"

    If you want to safely treat Person and all subclasses the same:

    me = NullProxy(Person(name="Alice"))
    me.role == None
    """

    def __init__(self, obj=None, default=None, parent=None):
        self.obj = obj
        self.default = default
        self.parent = parent

    def __call__(self, *args, **kw):
        if not hasattr(self.obj, '__call__'):
            raise TypeError("'{0!r}' object is not callable".format(self.obj))
        
        value = self.obj(*args, **kw)
        
        if value is None:
            return self.parent
        
        return value

    def __getattr__(self, name):
        try:
            return NullProxy(getattr(self.obj, name), self.default, self.obj)
        except AttributeError:
            return NullProxy(self.default, self.default, self.obj)

    def __setattr__(self, name, value):
        if self.obj is None:
            raise AttributeError("Can not write to null object.")

        setattr(self.obj, name, value)


class Borg(object):
    __collective__ = None

    def __init__(self):
        name = '_'.join(reversed([i.__name__ for i in type(self).mro()]))
        if Borg.__dict__.setdefault('__collective_{0}__'.format(name), None) is None:
            Borg.__dict__[name] = self.__dict__
        else:
            self.__dict__ = Borg.__dict__[name]

    def __eq__(self, other):
        return self.__dict__ is other.__dict__