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

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.