Yet another CaseInsensitiveDict implementation

caseinsensitivedict.py

class CaseInsensitiveDict(dict):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.__map = {}

        mapping = args[0] if len(args) else {}
        keys = set()
        if isinstance(mapping, dict):
            keys |= mapping.keys()
        elif isinstance(mapping, (list, tuple)):
            keys |= {key for key, *_ in mapping}
        else:
            raise ValueError('Mapping must be either a dict or a list/tuple')
        keys |= kwargs.keys()

        for key in keys:
            self.__map[self.__key_name(key)] = key

    @staticmethod
    def __key_name(item):
        return item.lower() if isinstance(item, str) else item

    def __getitem__(self, item):
        return dict.__getitem__(self, self.__map[self.__key_name(item)])

    def __setitem__(self, item, value):
        key = self.__key_name(item)
        if key in self.__map:
            # Delete old key before setting the new one to avoid duplicates
            dict.__delitem__(self, self.__map[key])
        self.__map[key] = item
        dict.__setitem__(self, item, value)


if __name__ == '__main__':
    a = CaseInsensitiveDict({'b': 'c'})
    a['A'] = 'b'
    assert a.keys() == {'b', 'A'}, a.keys()
    assert a['A'] == 'b'
    a['a'] = 'b'
    assert a['a'] == 'b'
    assert a.keys() == {'b', 'a'}, a.keys()
    a['B'] = 'c'
    assert a['B'] == 'c'
    assert a.keys() == {'B', 'a'}, a.keys()

    b = CaseInsensitiveDict([('A', 'b'), ('c', 'd')])
    b['a'] = 'b'
    assert b.keys() == {'a', 'c'}

    c = CaseInsensitiveDict(A='b', c='d')
    c['a'] = 'b'
    assert c.keys() == {'a', 'c'}, c.keys()

que gambeta