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 Fi         


        

Answer 1

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.