[Python-Dev] Dataclasses and correct hashability

[Guido van Rossum]

I'm not a fan, sorry.

On Tue, Feb 6, 2018 at 7:33 PM, Ethan Furman <ethan at stoneleaf.us> wrote:

> On 02/06/2018 06:48 PM, Guido van Rossum wrote:
>
> That seems a rare case (though I hadn't thought of it). I had thought of
>> the use case where you want a frozen type
>> without a hash; that you can presumably implement using
>>
>> def __hash__(self): raise TypeError("not hashable")
>>
>> We can do a similar thing to preserve the superclass __hash__ if it's
>> rare enough:
>>
>> def __hash__(self): return super().__hash__()
>>
>> If at all possible I'd like to kill the tri-state hash= flag -- the
>> amount of time spent creating and discussing the
>> huge table in the bpo issue are an indication of how much effort it would
>> take other people to understand it.
>>
>
> I think the biggest reason this has become so complicated is because we
> are refusing to use an Enum:
>
> class Hashable(Enum):
>     IF_SAFE = 1
>     ADD = 2
>     DEFER = 3
>     NONE = 4
>
> IF_SAFE is currently the False value.
> ADD is currently the True value
> DEFER means don't add one
> NONE means set __hash__ to None
>
> The only thing missing now is a setting to indicate that dataclass should
> do nothing if the class already has a __hash__ method -- possibly DEFER,
> although I think IF_SAFE can include "the class already has one, it's not
> safe to override it".
>
> If either of those use cases becomes annoyingly common we'll have to think
>> of something else.
>>
>
> Or we could solve it now and not have to deal with backwards-compatibility
> issues in the future.
>
> --
> ~Ethan~
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-- 
--Guido van Rossum (python.org/~guido)
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