Am 18.01.2013 12:56, schrieb Jean-Michel Pichavant: > You guessed right. But it took me a lot of time before jumping to that > conclusion, mostly because the code worked in the first place (it can with a > little bit of luck). > Now I'm extra careful about what I use as dict key, I was just wondering if > there was a reliable quick way to verify an object can be used as key. > > That brings me to another question, is there any valid test case where key1 > != key2 and hash(key1) == hash(key2) ? Or is it some kind of design flaw ?
That's a valid case. Hashing is not a permutation, it's (usually) surjective, not bijective. hash(key1) == hash(key2) is called a "hash collision". A good hashing algorithm should try to reduce the probability of collisions for likely keys. A secure algorithm should also make it as hard as possible to calculate deliberate collision. Collisions diminish the efficiency of a hash map from O(1) to O(n) and can be used for denial of service attacks. -- http://mail.python.org/mailman/listinfo/python-list
