Two Way Python dict

two_way_dict.py

class TwoWayDict(dict):
    def __setitem__(self, key, value):
        dict.__setitem__(self, key, value)
        if value in self:
            if isinstance(dict.__getitem__(self, value), list):
                self[value].append(key)
        else:
            dict.__setitem__(self, value, [key])

In [88]: t = TwoWayDict()

In [89]: t['a'] = 1

In [90]: t['b'] = 1

In [91]: t['c'] = 1

In [92]: t
Out[92]: {1: ['a', 'b', 'c'], 'a': 1, 'b': 1, 'c': 1}