class Attr_Diff(dict):
    """
    Represents a change to note attributes, where nodes can represent
    either logical nodes or logical links, and attributes can be added,
    changed or removed
    
    Example:
            attr_diff = {'__type_node' : {n_id: {'__attr_write': {'attr_0': 0,
                                                                  'attr_1': 'a'},
                                                '__attr_remove': ['attr_2'] }}
                         '__type_link' : {l_id: ... }
                        }
    """
    def __init__(self):
        self['__type_node'] = {}
        self['__type_link'] = {}

    def init_node_attr_diff(self, n_id):
        ret = {'__attr_write': {},
               '__attr_remove': []}
        self['__type_node'][n_id] = ret
        return ret

    @staticmethod
    def from_json_dict(json_dict):
        ret = Attr_Diff()
        for obj_type in ret.keys():
            obj_ad_set = json_dict.get(obj_type)
            if None != obj_ad_set:
                for o_id, ad in obj_ad_set.items():
                    if None != ad.get('__attr_write'):
                        for k, v in ad['__attr_write'].items():
                            ret.add_node_attr_write(o_id, k, v)
                    if None != ad.get('__attr_remove'):
                        for k in ad['__attr_remove']:
                            ret.add_node_attr_rm(o_id, k)
        return ret

    @property
    def type__node(self):
        return self['__type_node']

    @property
    def type__link(self):
        return self['__type_link']

    def add_node_attr_write(self, n_id, attr_name, attr_val):

        assert 'id' != attr_name.lower(), 'Attr_Diff: attempt to write to \'id\' attribute'

        n_attr_diff = self['__type_node'].get(n_id)
        if None == n_attr_diff:
            n_attr_diff = self.init_node_attr_diff(n_id)
        n_attr_diff['__attr_write'][attr_name] = attr_val

    def add_node_attr_rm(self, n_id, attr_name):
        n_attr_diff = self['__type_node'].get(n_id)
        if None == n_attr_diff:
            n_attr_diff = self.init_node_attr_diff(n_id)
        n_attr_diff['__attr_remove'].append(attr_name)

    def add_link_attr_write(self, l_id, attr_name, attr_val):
        assert False, 'unimplemented'

    def add_link_attr_rm(self, l_id, attr_name):
        assert False, 'unimplemented'

class Topo_Diff(object):
    """
    Represents a change to the graph topology
    """
    def __init__(self, link_set_rm=[],
                       node_set_rm=[],
                       node_set_add=[],
                       link_set_add=[]):

        self.link_set_rm = link_set_rm
        self.node_set_rm = node_set_rm
        self.node_set_add = node_set_add
        self.link_set_add = link_set_add

    def __str__(self):
        return __name__ + ': ' + ', '.join('%s: %s' % (k, v) for k, v in self.__dict__.items())

    def check_validity(self, topo_diff_dict):
        """
        Topo_Diff may represent invalid operations, eg. adding a link while
         removing it's end-point - this stub should check for that
        """
        pass

    @staticmethod
    def from_json_dict(json_dict):
        """
        construct from dict - no node/link constructor set must be provided
        """
        ret = Topo_Diff()

        # merge keys - this allows constructor argument omission (link_set_rm,
        # node_set_rm, etc.) such as when constructing from POST JSON data
        for k, _ in ret.__dict__.items():
            v = json_dict.get(k)
            if None != v:
                ret.__dict__[k] = v
        return ret