Hi. I've recently stumbled on some at least to me unexpected behavior with zope.keyreference. For a persistent object it generates a unique key using: hash((database_name, oid)) where hash is Python's built-in hash function. Reading the documentation I assumed that a keyreference for the same object (as identified by database name and oid) should be stable and always produce the same result. This isn't always true, when you look up persisted keyreference data, upgrade your software versions and compare it to a new calculation. Python's hash function is only stable inside the same Python version and 32/64 bit combination. The same input in a 32bit Python 2.6 and 64bit Python 2.6 produces different results, as both try to use the maximum available integer space and thus a 64bit Python generates keys above the 32int range. As a simple example "hash(('main', 1)) > 2**32" is True in a 64bit Python and False in a 32bit Python. The internal hash implementation seems to have been pretty stable in all the latest Python versions up to 3.1. So the algorithm produces the same results for all 32bit version of Python 2.x to 3.1 and 64bit respectively. But as far as I understand this isn't guaranteed to be the case for future versions. Does anyone else see a problem with this? Should keyreference use a different hash algorithm? Hanno