[ZODB-Dev] BTree sets and unhashable objects

[Tim Peters]

[Tim]
>> I don't know which Wiki that refers to (have a URL handy?), but it's
>> wrong: BTree operations (which include OOSets) never ask for an
>> object's hash code (Python dicts do, BTrees do not).

[Roché Compaan]
> http://www.zope.org/Members/ajung/BTrees/DataTypes

Thanks!  I added comments to the page.

> Actually it doesn't say that sets can only work with hashable objects
> - I assumed that this was the problem because the wiki does say that
> OOBTrees maps hashable objects to hashable objects.

It's still wrong <wink>.  Keys don't have to be hashable, but do need a
consistent total ordering; values don't have to be hashable or comparable.

...

>> The problem is due to something else.

> So this might be a bug?

Probably not in the BTree code, although that's possible.  It's really not
possible to say more here now, since we haven't seen any part of your code.

>> The "real rules" for BTrees are here:
>>
>>     http://www.zope.org/Wikis/ZODB/guide/node6.html
>>
>> esp. section 5.3.1, "Total Ordering and Persistence".

> I'll have a look tonight to see if I meet all the criteria - if I do
> then I will send some more info to determine if I've hit a bug or not.

The easiest way to get in trouble is to use instances of a user-defined
(app-defined) class, which class doesn't define a __cmp__ method.