frozendict (v0.1)

[Jonas H.]

On 10/08/2010 02:23 AM, kj wrote:
> I imagine that frozenset is better than sorted(tuple(...)) here,
> but it's not obvious to me why.

dicts are unsorted. That means their item-order is undefined. So are sets.

If you want a hash that is independent from the order of items, you 
could ensure the items are always in the same order when you do the 
hashing; or you could use a hashing algorithm that ignore item order.

As creating a `frozenset` is probably more efficient than sorting, that 
is the preferred solution.

Here's my implementation suggestion:

class frozendict(dict):
     def _immutable_error(self, *args, **kwargs):
         raise TypeError("%r object is immutable" % self.__class__.__name__)

     __setitem__ = __delitem__ = clear = pop \
     = popitem = setdefault = update = _immutable_error

     def __hash__(self):
         return hash(frozenset(self.iteritems()))

Only 9 lines :-)

Jonas