[Zope-dev] zope.keyreference hashes vs. 32/64bit

[Hanno Schlichting]

Hi.

On Sat, Aug 28, 2010 at 8:47 PM, Jim Fulton <jim at zope.com> wrote:
> On Sat, Aug 28, 2010 at 12:17 PM, Hanno Schlichting <hanno at hannosch.eu> wrote:
>> I've recently stumbled on some at least to me unexpected behavior with
>> zope.keyreference.
>
> Specifically, zope.keyreference.persistent, I assume.

Yes.

>> Does anyone else see a problem with this? Should keyreference use a
>> different hash algorithm?
>
> Potentially, yes.  In current practice, I don't think so.
>
> When a key reference is uses as a BTree key, its comparison function,
> rather than it's hash is used.
>
> If a key reference hash was used as a persistent key, then this would
> definitely be a problem.
>
> Note that in a dictionary or PersistentMapping, the hash isn't
> saved persistently. The object is saves as a collection of items and the
> hashes are recomputed on unpickling.

Ah right. This makes it less likely to be a problem in practice.

> I'm in favor of someone coming up with a stable hash to
> avoid future pitfalls.
>
> It's sad that Python's hash isn't stable across Python versions
> and architectures. Is this documented? If so, It's a missfeature.
> If not, perhaps it should be reported as a bug.

The official Python documentation doesn't specify anything explicitly,
but it also doesn't describe the algoritm or state that it's stable.

You do immediately find http://effbot.org/zone/python-hash.htm
googling for "python hash" though. This notes that the algorithm
changed in Python 2.4. Looking at the NEWS file of Python, the hash
algorithm has again changed in Python 3.2 alpha 1 referencing issue
8188.

Hanno