[Python-Dev] Dataclasses and correct hashability
Guido van Rossum
guido at python.org
Tue Feb 6 14:18:20 EST 2018
We may be in violent agreement.
I propose *not* to add a way to *disable* hashing when the rest of the
flags to @dataclass() would indicate that it's safe to add a __hash__
I propose that with @dataclass(unsafe_hash=False) (the default), a __hash__
method is added when the following conditions are true:
- frozen=True (not the default)
- compare=True (the default)
- no __hash__ method is defined in the class
If we instead use @dataclass(unsafe_hash=True), a __hash__ will be added
regardless of the other flags, but if a __hash__ method is present, an
exception is raised.
Other values (e.g. unsafe_hash=None) are illegal for this flag.
Note that the the hash= flag to the field() function is unchanged from
what's currently in PEP 557 or in the implementation in 3.7.0b1. In
particular, the generated __hash__ method will disregard fields declared
using field(hash=False). It will also disregard fields declared using
On Tue, Feb 6, 2018 at 10:11 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
> On 02/06/2018 09:38 AM, Guido van Rossum wrote:
> Where do you get the impression that one would have to explicitly request
>> __hash__ if frozen=True is set? To the
>> contrary, my proposal is for @dataclass to automatically add a __hash__
>> method when frozen=True is set. This is what the
>> code currently released as 3.7.0b1 does if hash=None (the default).
> Which is my issue with the naming -- although, really, it's more with the
> parameter/argument: in a hand-written class,
> __hash__ = None
> means the object in is not hashable, but with the decorator:
> @dataclass(..., hash=None, ...)
> it means something else.
> My preference for "fixing" the issue:
> 1) make the default be a custom object (not None), so that `hash=None`
> means disable hashing
> 2) change the param name -- maybe to `add_hash` (I agree with D'Aprano
> that `unsafe_hash` can be misleading)
> Python-Dev mailing list
> Python-Dev at python.org
> Unsubscribe: https://mail.python.org/mailman/options/python-dev/guido%
--Guido van Rossum (python.org/~guido)
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Python-Dev