New submission from Barry A. Warsaw: If you have a custom object that implements __hash__() and it returns a value wider than Py_ssize_t, built-in hash() on the object will truncate information. This is because hash() takes the value returned by obj.__hash__() and coerces it through PyLong_FromSsize_t(). This can cause object hashes to have different values on 64bit and 32bit machines, e.g. on 64bit Linux where Py_ssize_t is 8 bytes wide vs. 32bit Linux where it is 4 bytes wide.
This may be perfectly reasonable from an implementation point of (ref: issue9778) but it is surprising since it is not documented. This size constraint on object hashes should be documented. from ctypes import * class Foo: def __hash__(self): return 0x1332a6000000000 print(hash(Foo()), sizeof(c_ssize_t)) ---64bit Ubuntu 13.10--- $ python3.3 hashex.py 86459409655398400 8 ---32bit Ubuntu 13.10--- $ python3 hashex.py 40260800 4 ---------- assignee: docs@python components: Documentation messages: 193014 nosy: barry, docs@python priority: normal severity: normal status: open title: hash() and __hash__() do not document their size constraints type: behavior versions: Python 3.2, Python 3.3, Python 3.4 _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue18440> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com