[ZODB-Dev] Weird KeyError with OOBTree

[Tres Seaver]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Hanno Schlichting wrote:
> On Mon, Aug 16, 2010 at 12:14 PM, Pedro Ferreira
> <jose.pedro.ferreira at cern.ch> wrote:
>> Could this be some problem with using persistent objects as keys in a BTree?
>> Some comparison problem?
> 
> I'm not entirely sure about this, but I think using persistent objects
> as keys isn't supported. Looking at the code, I doubt using anything
> expect simple types like unicode strings or tuples of simple types
> will work without further work.
> 
>>From what I can see in the code, BTree's use functions like
> PyObject_Compare to compare different keys. Persistent doesn't
> implement any special compare function and falls back to the standard
> hash algorithm for an object. This happens to be its memory address.
> The memory address obviously changes over time and the same address
> gets reused for different objects.
> 
> I think implementing a stable hash function for your type could make
> this work though.
> 
> The ZODB gods correct me please :)

Btrees require "comparability", rather than "hashability":  your
persistent type needs to define a total ordering[1], which typically
means defining '__cmp__' for your class.  You could also define just
'__eq__' and '__lt__', but '__cmp__' is slightly more efficient.


[1]http://www.zodb.org/documentation/guide/modules.html#total-ordering-and-persistence


Tres.
- --
===================================================================
Tres Seaver          +1 540-429-0999          tseaver at palladion.com
Palladion Software   "Excellence by Design"    http://palladion.com
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.9 (GNU/Linux)
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org

iEYEARECAAYFAkxpK18ACgkQ+gerLs4ltQ5gIwCfQDXxLwxpWrghFtRF76MBxesS
RtcAnA3+yv+6+XfDRkPBBZcZpoqZ6mO0
=ygVN
-----END PGP SIGNATURE-----