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POImmutable dictionary, only use as a key for another dictionary
 

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    <p>I had the need to implement a hashable dict so I could use a dictionary as a key for another dictionary.</p> <p>A few months ago I used this implementation: <a href="https://stackoverflow.com/questions/1151658/python-hashable-dicts">Python hashable dicts</a></p> <p>However I got a notice from a colleague saying 'it is not really immutable, thus it is not safe. You can use it, but it does make me feel like a sad Panda'.</p> <p>So I started looking around to create one that is immutable. I have no need to compare the 'key-dict' to another 'key-dict'. Its only use is as a key for another dictionary.</p> <p>I have come up with the following:</p> <pre><code>class HashableDict(dict): """Hashable dict that can be used as a key in other dictionaries""" def __new__(self, *args, **kwargs): # create a new local dict, that will be used by the HashableDictBase closure class immutableDict = dict(*args, **kwargs) class HashableDictBase(object): """Hashable dict that can be used as a key in other dictionaries. This is now immutable""" def __key(self): """Return a tuple of the current keys""" return tuple((k, immutableDict[k]) for k in sorted(immutableDict)) def __hash__(self): """Return a hash of __key""" return hash(self.__key()) def __eq__(self, other): """Compare two __keys""" return self.__key() == other.__key() # pylint: disable-msg=W0212 def __repr__(self): """@see: dict.__repr__""" return immutableDict.__repr__() def __str__(self): """@see: dict.__str__""" return immutableDict.__str__() def __setattr__(self, *args): raise TypeError("can't modify immutable instance") __delattr__ = __setattr__ return HashableDictBase() </code></pre> <p>I used the following to test the functionality:</p> <pre><code>d = {"a" : 1} a = HashableDict(d) b = HashableDict({"b" : 2}) print a d["b"] = 2 print a c = HashableDict({"a" : 1}) test = {a : "value with a dict as key (key a)", b : "value with a dict as key (key b)"} print test[a] print test[b] print test[c] </code></pre> <p>which gives:</p> <blockquote> <p>{'a': 1}<br> {'a': 1}<br> value with a dict as key (key a)<br> value with a dict as key (key b)<br> value with a dict as key (key a) </p> </blockquote> <p>as output </p> <p>Is this the 'best possible' immutable dictionary that I can use that satisfies my requirements? If not, what would be a better solution?</p>
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    1. COA slightly nicer method would be ``tuple(sorted(immutableDict.items()))`` (or ``iteritems()`` pre 3.x). Also, just as a note, I'd go for ``FrozenDict`` as a name given the ``frozenset`` class that exists in Python by default, just for naming consistency - not that it really matters.
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      1. USGareth Latty
 

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