Define a python dictionary with immutable keys but mutable values

Question

Well, the question is in the title: how do I define a python dictionary with immutable keys but mutable values? I came up with this (in python 2.x):

class FixedDict(dict):
    """
    A dictionary with a fixed set of keys
    """

    def __init__(self, dictionary):
        dict.__init__(self)
        for key in dictionary.keys():
            dict.__setitem__(self, key, dictionary[key])

    def __setitem__(self, key, item):
        if key not in self:
            raise KeyError("The key '" +key+"' is not defined")
        dict.__setitem__(self, key, item)

but it looks to me (unsurprisingly) rather sloppy. In particular, is this safe or is there the risk of actually changing/adding some keys, since I'm inheriting from dict? Thanks.

Answer 1

Consider proxying dict instead of subclassing it. That means that only the methods that you define will be allowed, instead of falling back to dict's implementations.

class FixedDict(object):
        def __init__(self, dictionary):
            self._dictionary = dictionary
        def __setitem__(self, key, item):
                if key not in self._dictionary:
                    raise KeyError("The key {} is not defined.".format(key))
                self._dictionary[key] = item
        def __getitem__(self, key):
            return self._dictionary[key]

Also, you should use string formatting instead of + to generate the error message, since otherwise it will crash for any value that's not a string.

Answer 2

The problem with direct inheritance from dict is that it's quite hard to comply with the full dict's contract (e.g. in your case, update method won't behave in a consistent way).

What you want, is to extend the collections.MutableMapping:

import collections

class FixedDict(collections.MutableMapping):
    def __init__(self, data):
        self.__data = data

    def __len__(self):
        return len(self.__data)

    def __iter__(self):
        return iter(self.__data)

    def __setitem__(self, k, v):
        if k not in self.__data:
            raise KeyError(k)

        self.__data[k] = v

    def __delitem__(self, k):
        raise NotImplementedError

    def __getitem__(self, k):
        return self.__data[k]

    def __contains__(self, k):
        return k in self.__data

Note that the original (wrapped) dict will be modified, if you don't want that to happen, use copy or deepcopy.