>>> 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
Python-Dev@python.org
https://mail.python.org/mailman/listinfo/python-dev
Unsubscribe:
https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
