[Toby Dickenson] > ... > ZODB's BTrees work in a similar way but use the regular python comparison > function, and the lack of a guarantee of a total ordering can be a liability. > Described here in 2002, but I think same is true today: > http://mail.zope.org/pipermail/zodb-dev/2002-February/002304.html
There's a long discussion of this in the ZODB Programming Guide, subsection "5.3.1 Total Ordering and Persistence" on this page: http://www.zope.org/Wikis/ZODB/FrontPage/guide/node6.html That talks about more than the referenced ZODB thread covered. Persistence adds more twists, such as that the total ordering among keys must remain the same across time (including across Python releases; for example, how None compares to objects of other types has changed across releases). -- http://mail.python.org/mailman/listinfo/python-list