Bugs item #660098, was opened at 2002-12-30 13:39 Message generated for change (Comment added) made by danielhs You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=660098&group_id=5470
Please note that this message will contain a full copy of the comment thread, including the initial issue submission, for this request, not just the latest update. Category: None Group: None Status: Closed Resolution: Rejected Priority: 5 Submitted By: Thomas Heller (theller) Assigned to: Guido van Rossum (gvanrossum) Summary: New style classes and __hash__ Initial Comment: New style classes obviously inherit a __hash__ implementation from object which returns the id. Per default this allows using instances as dictionary keys, but usually with the wrong behaviour, because most often user classes are mutable, and their contained data should be used to calculate the hash value. IMO one possible solution would be to change typeobject.c:object_hash() to raise TypeError, and change all the immutable (core) Python objects to use _Py_HashPointer in their tp_hash slot. ---------------------------------------------------------------------- Comment By: Daniel (danielhs) Date: 2006-10-06 17:29 Message: Logged In: YES user_id=1609821 I don't know if this is the right answer or not, but, this post seems to indicate that the fix would break Jython. But a later post in the thread notes that since the latest version of Jython is only version 2.1 this shouldn't be an issue. First post: http://mail.python.org/pipermail/python-list/2004-December/257637.html Second post: http://mail.python.org/pipermail/python-list/2004-December/257690.html Daniel ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2006-10-06 17:19 Message: Logged In: YES user_id=6380 I don't recall what was wrong with the patch. I do know that I fixed this in Python 3000; but the fix there was only possible due to other unrelated fixes that couldn't possibly be backported. I propose to leave this broken until Py3k, so let's close this bug. ---------------------------------------------------------------------- Comment By: Daniel (danielhs) Date: 2006-10-05 22:22 Message: Logged In: YES user_id=1609821 Still doesn't work as expected in 2.5. Just wanted to bump along since I noticed this bug today, and found this bug report (which hasn't changed in nearly 3 years). ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2003-12-22 16:04 Message: Logged In: YES user_id=6380 Anybody see a reason why I shouldn't check this in? See python-dev discussion. ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2003-12-05 13:30 Message: Logged In: YES user_id=6380 Here's the patch I am thinking of. ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2003-12-05 13:06 Message: Logged In: YES user_id=6380 I wonder if the solution could be as simple as removing the tp_hash slot from the object class? I just tried that and it passes the entire test suite, as well as the tests that Tim added to the patch. The trick is that PyObject_Hash() has a fallback which does the right thing. And when the base object class doesn't set tp_compare or tp_richcompare, I think it should be allowed not to set tp_hash either. ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2003-05-11 09:41 Message: Logged In: YES user_id=6380 Oops, not so fast. This also makes object.__hash__() calls fail when it is explicitly invoked, e.g. when a class overrides __eq__ to print a message and then call the base class __eq__, it must do the same for __hash__, but object.__hash__ will still fail in this case. I'll think of a fix for that. ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2003-05-11 06:16 Message: Logged In: YES user_id=6380 OK, feel free to check it in. ---------------------------------------------------------------------- Comment By: Tim Peters (tim_one) Date: 2003-05-11 00:39 Message: Logged In: YES user_id=31435 The patch seems fines to me. I've attached a new patch, combining yours with new tests in test_class.py. ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2003-05-09 14:04 Message: Logged In: YES user_id=6380 A trick similar to what we do in object_new might work. There, we raise an error if the tp_init slot is the default function (object_init) and any arguments are passed. I propose that object_hash checks that tp_compare and tp_richcompare are both NULL. I'm attaching a patch -- let me know if that works. ---------------------------------------------------------------------- Comment By: Tim Peters (tim_one) Date: 2003-05-09 13:54 Message: Logged In: YES user_id=31435 >>> class C: # classic class complains ... __cmp__ = lambda a, b: 0 ... >>> {C(): 1} Traceback (most recent call last): File "<stdin>", line 1, in ? TypeError: unhashable instance >>> class C(object): # new-style class does not complain ... __cmp__ = lambda a, b: 0 ... >>> {C(): 1} {<__main__.C object at 0x007F6970>: 1} >>> That was under current CVS. I see the same behavior in 2.2.3, so this isn't new. About Thomas's original report, I don't agree -- the default behavior is very useful. The rule I've always lived by is that, to be usable as a dict key, an instance's class must either: 1. Implement none of {__cmp__, __eq__, __hash__}. or 2. Implement __hash__ and (at least) one of {__cmp, __eq__}. Classic classes still work this way. New-style classes don't appear to outlaw any combination here. ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2003-05-09 13:44 Message: Logged In: YES user_id=6380 Yes, please paste an example here. ---------------------------------------------------------------------- Comment By: Jeremy Hylton (jhylton) Date: 2003-05-09 13:32 Message: Logged In: YES user_id=31392 Currently, a new-style class that defines __cmp__ but not __hash__ is usable as a dictionary key. That seems related to this bug. Should I paste the example here and bump the priority? Or should I open a separate bug report? ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2003-02-11 18:01 Message: Logged In: YES user_id=6380 I spent an afternoon looking into this, and I can't see an easy solution. The idea of only inheriting __hash__ together with certain other slots is really flawed; it may be better if object DIDN'T define a default implementation for __hash__, comparisons (both flavors), and other things, or maybe the default __hash__ should raise an exception when the comparisons are not those inherited from object, or maybe PyType_Ready should insert a dummy __hash__ when it sees that you redefine __eq__, or... I really don't know. I'm going to sleep on this some more, and lower the priority. You can always get the right behavior by explicitly defining __hash__. ---------------------------------------------------------------------- Comment By: Thomas Heller (theller) Date: 2002-12-30 13:50 Message: Logged In: YES user_id=11105 You mean at the end of the inherit_slots() function? For my extension which I'm currently debugging, tp_compare, tp_richcompare, and tp_hash are inherited from base, but only tp_hash is != NULL there. ---------------------------------------------------------------------- Comment By: Guido van Rossum (gvanrossum) Date: 2002-12-30 13:44 Message: Logged In: YES user_id=6380 There seems to be code that tries to inherit tp_hash only when tp_compare and tp_richcompare are also inherited, but it seems to be failing. ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=105470&aid=660098&group_id=5470 _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
