[Python-Dev] Status of the fix for the hash collision vulnerability

Stefan Behnel stefan_ml at behnel.de
Sun Jan 15 15:30:59 CET 2012

Terry Reedy, 14.01.2012 06:43:
> On 1/13/2012 8:58 PM, Gregory P. Smith wrote:
>> It is perfectly okay to break existing users who had anything depending
>> on ordering of internal hash tables. Their code was already broken.
> Given that the doc says "Return the hash value of the object", I do not
> think we should be so hard-nosed. The above clearly implies that there is
> such a thing as *the* Python hash value for an object. And indeed, that has
> been true across many versions. If we had written "Return a hash value for
> the object, which can vary from run to run", the case would be different.

Just a side note, but I don't think hash() is the right place to document
this. Hashing is a protocol in Python, just like indexing or iteration.
Nothing keeps an object from changing its hash value due to modification,
and that would even be valid in the face of the usual dict lookup
invariants if changes are only applied while the object is not referenced
by any dict. So the guarantees do not depend on the function hash() and may
be even weaker than your above statement.


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