[ZODB-Dev] odd KeyError in OOSets

[Tim Peters]

[Nicholas Henke, on ClassmaskObject versus __cmp__]
> It derives from IndexedObject from IndexedCatalog -- I checked that
> class, and __cmp__ is not defined anywhere. I am not sure if it matters,
> __hash__ is defined to return the objects 'id' which is id(self) taken
> during __init__.

While __hash__ is important for Python dicts, __hash__ isn't used by ZODB
Sets, TreeSets, Buckets or BTrees.  They store stuff in sorted order, and
require a __cmp__ that defines a consistent ordering among members (for Sets
and TreeSets) or keys (for Buckets and BTrees).  Instances of a Python class
that doesn't define __cmp__ get a default __cmp_, which compares the
instances' locations in memory (it compares their memory addresses).  This
is a consistent total ordering across a single program run, but must never
be used for persistent objects -- the memory address they land at when
loaded from a database is accidental.  Unfortunately, nothing in ZODB can
detect the error when you end up using a bad comparison function, except
that the results are disastrous.

Short course:  if there isn't a carefully designed __cmp__ in this class
hierarchy, you simply can't use instances of this class as elements of
persistent Sets or TreeSets, or as keys in persistent Buckets or BTrees.