[Python-Dev] Dataclasses and correct hashability

[Guido van Rossum]

It appears Eric and I are the only ones in favor of keeping the current
behavior. But I still am not convinced by all the worries about "attractive
nuisances" and all the other bad names this feature has been called. We
don't know that any of the doomsday scenarios will happen. In my
experience, usually once something is rolled out the set of issues that are
*actually* raised is entirely different from the things its designers were
worried about.

Please don't commit a change to roll this back without checking in with me;
I have some misgivings about the problem being raised here that I still
need to think through more carefully. In the meantime, please try to use
dataclasses with 3.7b1!

On Fri, Feb 2, 2018 at 10:25 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

>
>
> On 3 Feb. 2018 1:09 am, "Eric V. Smith" <eric at trueblade.com> wrote:
>
>
> The problem with dropping hash=True is: how would you write __hash__
> yourself? It seems like a bug magnet if you're adding fields to the class
> and forget to update __hash__, especially in the presence of per-field
> hash=False and eq=False settings. And you'd need to make sure it matches
> the generated __eq__ (if 2 objects are equal, they need to have the same
> hash value).
>
>
> I think anyone that does this needs to think *very* carefully about how
> they do it, and offering both "hash=True" and "frozen=True" is an
> attractive nuisance that means people will write "hash=True" when what they
> wanted was "frozen=True".
>
> In particular, having to work out how write a maintainable "__hash__" will
> encourage folks to separate out the hashed fields as a separate frozen
> subrecord or base class.
>
> If this proves to be an intolerable burden then the short hand spelling
> could be added back in 3.8, but once we ship it we're going to be stuck
> with explaining the interactions indefinitely.
>
> Cheers,
> Nick.
>
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-- 
--Guido van Rossum (python.org/~guido)
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