It must be any instance of an ExtensionClass. I think Python instances used to be true-for-callable as well, but that must have been fixed. Python 2.1.3 (#5, Aug 15 2002, 10:41:31) [GCC 2.95.3 19991030 (prerelease)] on linux2 Type "copyright", "credits" or "license" for more information. >>> from ZODB.PersistentList import * >>> callable(PersistentList()) 1 Jeremy