On Mon, 18 Mar 2013 18:31:33 +1100, Chris Angelico wrote: > On Mon, Mar 18, 2013 at 6:26 PM, Frank Millman <fr...@chagford.com> > wrote: >> Hi all >> >> I know that you cannot rely on the order of keys in a dictionary, and I >> am not attempting to do so. >> >> Nevertheless, the following surprised me. A program creates a >> dictionary with a known set of keys. I would have thought that multiple >> runs of the program would return the keys in the same sequence. As you >> can see, the results appear to be totally random. >> >> Just out of interest, I would appreciate an explanation. > > Mainly, it's just something you utterly cannot depend on, so it's > allowed to vary based on the phase of the moon, the position of your > hard drive platters, or anything else it likes. The hashing is actually > randomized in recent versions of Python as a defense against a denial of > service attack by generating hash collisions; that's most likely what > you're seeing there. The dictionary object is not deterministic to that > extent. :)
Correct. If you try the same in Python 3.2 instead of 3.3, you will get the same order each time. You can read more about the dictionary hashing attack here: http://mail.python.org/pipermail/python-dev/2011-December/115116.html http://bugs.python.org/issue13703 http://bugs.python.org/issue14621 The current status is that Python's dict hashes are improved, but still leak enough information that attackers can cause a Denial Of Service attack with just a bit more effort than before, but work is ongoing to solve this issue. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
