Stupid Immutable Dict

stupid_immutable_dict.py

def imdict(dct):
    """Construct an immutable dict, based on a normal one.

    """

    # Warning: It's possible for others to get at these closed-over variables
    # by inspecting frames, and then modifying them. Suggestions welcome (other
    # than implementing a new hash table with tuples from scratch)
    items = dct.items()
    lookup = dict([(item[0], index) for index, item in enumerate(items)])

    class ImmutableDict(tuple):
        def __new__(klass, dct):
            values = [x[1] for x in items]
            return super(ImmutableDict, klass).__new__(klass, values)

        def __getitem__(self, key):
            field = lookup[key]
            return super(ImmutableDict, self).__getitem__(field)

    return ImmutableDict(dct)

if __name__ == '__main__':
    d = imdict({"a": 1, "b": 2})

    assert d['a'] == 1
    assert d['b'] == 2
    try:
        d['b'] = 3
    except TypeError:
        print "good: can't set attributes"