>>> 2) Change the default value for "hash" from "None" to "False". This might >>> take a little effort because there is currently an oddity where setting >>> hash=False causes it to be hashable. I'm pretty sure this wasn't intended >>> ;-) >> I haven't looked at this yet. > > I think the hashing logic explained in > https://bugs.python.org/issue32513#msg310830 is correct. It uses hash=None as > the default, so that frozen=True objects are hashable, which they would not > be if hash=False were the default.
Wouldn't it be simpler to make the options orthogonal? Frozen need not imply hashable. I would think if a user wants frozen and hashable, they could just write frozen=True and hashable=True. That would more explicit and clear than just having frozen=True imply that hashability gets turned-on implicitly whether you want it or not. > If there's some case there that you disagree with, I'd be interested in > hearing about it. > > That logic is what is currently scheduled to go in to 3.7 beta 1. I have not > updated the PEP yet, mostly because it's so difficult to explain. That might be a strong hint that this part of the API needs to be simplified :-) "If the implementation is hard to explain, it's a bad idea." -- Zen If for some reason, dataclasses really do need tri-state logic, it may be better off with enum values (NOT_HASHABLE, VALUE_HASHABLE, IDENTITY_HASHABLE, HASHABLE_IF_FROZEN or some such) rather than with None, True, and False which don't communicate enough information to understand what the decorator is doing. > What's the case where setting hash=False causes it to be hashable? I don't > think that was ever the case, and I hope it's not the case now. Python 3.7.0a4+ (heads/master:631fd38dbf, Jan 28 2018, 16:20:11) [GCC 7.2.0] on darwin Type "copyright", "credits" or "license()" for more information. >>> from dataclasses import dataclass >>> @dataclass(hash=False) class A: x: int >>> hash(A(1)) 285969507 I'm hoping that this part of the API gets thought through before it gets set in stone. Since dataclasses code never got a chance to live in the wild (on PyPI or some such), it behooves us to think through all the usability issues. To me at least, the tri-state hashability was entirely unexpected and hard to debug -- I had to do a close reading of the source to figure-out what was happening. Raymond _______________________________________________ Python-Dev mailing list [email protected] https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com