Read-only dict

from collections import Mapping

class FrozenDict(Mapping):
    """A dictionary that does not support item assignment after initialization
    >>> fd = FrozenDict(a=1, b=2)
    >>> fd["a"]
    1
    >>> fd["a"] = 777  # doctest: +IGNORE_EXCEPTION_DETAIL
    Traceback (most recent call last):
    ...
    TypeError: 'FrozenDict' object does not support item assignment
    >>> sorted(fd.items())
    [('a', 1), ('b', 2)]
    >>> fd.keys()
    ['a', 'b']
    >>> fd
    FrozenDict('a'=1, 'b'=2)
    """
    def __init__(self, **kwargs):
        self.__kwargs = kwargs
        self.__init__ = None
        self.__next__ = self.next
        self.__counter = 0
    def __repr__(self):
        kwargs = ["%r=%r" % (k, v) for k, v in sorted(self.__kwargs.items())]
        return "%s(%s)" % (self.__class__.__name__, ", ".join(kwargs))
    def __getitem__(self, key):
        return self.__kwargs[key]
    def __iter__(self):
        return self
    def next(self):
        try:
            value = self.__kwargs.items()[self.__counter][0]
        except IndexError:
            raise StopIteration
        self.__counter += 1
        return value
    def __len__(self):
        return len(self.__kwargs)

class ChillyDict(dict):
    """A dictionary that does not support item assignment after initialization
    >>> cd = ChillyDict(a=1, b=2)
    >>> cd["a"]
    1
    >>> cd["a"] = 777  # doctest: +IGNORE_EXCEPTION_DETAIL
    Traceback (most recent call last):
    ...
    NotImplementedError: 'ChillyDict' object does not support item assignment
    >>> sorted(cd.items())
    [('a', 1), ('b', 2)]
    >>> cd.keys()
    ['a', 'b']
    >>> cd
    ChillyDict('a'=1, 'b'=2)
    """
    def __init__(self, **kwargs):
        self.__kwargs = kwargs
        super(ChillyDict, self).__init__(**self.__kwargs)
        self.__init__ = None
    def __repr__(self):
        kwargs = ["%r=%r" % (k, v) for k, v in sorted(self.__kwargs.items())]
        return "%s(%s)" % (self.__class__.__name__, ", ".join(kwargs))
    def __not_implemented(self, *args, **kwargs):
        msg = "'%s' object does not support item assignment"
        raise NotImplementedError, msg % self.__class__.__name__
    __setitem__ = __delitem__ = update = pop = popitem = clear = __not_implemented

if __name__ == "__main__":
    import doctest
    doctest.testmod()