subclassing dict in python.

gistfile1.txt

class AttrStorage(dict):

    def __init__(self, fields=None, **kwargs):
    """ only keys defined in fields are allowed """
        self.fields = ()
        if fields:
            self.fields = fields
        super(AttrStorage, self).__init__(kwargs)
        
    def __getitem__(self, name):
        val =  super(AttrStorage, self).get(name)
        if val:
            return val
        raise KeyError(name)
        
    def __setitem__(self, k, v):
    """ Additionaly restrict changing the value once set """
        if len(self.fields) and k not in self.fields:
            raise AttributeError("{0}: not allowed".format(k))
        try:
            self.__getitem__(k)            
        except KeyError:
            pass
        else:
            raise RuntimeError('Read-only attribute: {0}'.format(k))
        return super(AttrStorage, self).__setitem__(k, v)