It is an invariant of the ZCatalog's FieldIndex that the items are sorted. However python does not guarantee that the sort order of differently-typed objects will not change. The language definition says: 'the choice whether one object is considered smaller or larger than another one is made arbitrarily but consistently within one execution of a program' http://www.python.org/doc/current/ref/comparisons.html Are FieldIndexes relying on undefined behavior? Toby Dickenson tdickenson@geminidataloggers.com