Just wanted to add my 5 cents here. I am a bit surprised how people are scared by adding `__hash__` to mutable classes. >From my experience it is quite normal, I was always thinking about `hash()` as hashing a _value_, and never as hashing _identity_, if I would need the latter, there is a different function for this, `id()`. Moreover, I often did this in situations where dataclasses would be useful: a class with several fields, necessary dunders, and few other methods (record-like classes). My motivation was mostly speed-up by memorization. To be fair my use cases were mostly about some tree-like strictures, but this is probably a coincidence.
FWIW it is not a super-safe practice, but neither super-dangerous. -- Ivan On 5 February 2018 at 22:56, Nick Coghlan <ncogh...@gmail.com> wrote: > On 6 February 2018 at 03:47, Guido van Rossum <gu...@python.org> wrote: > > If there's going to be an API for it, it should be in the class, not > > something that mutates the class afterwards. > > Something I realised after posting the __class__ setting idea is that > you can actually use a comparable trick to inject an unsafe hash from > the frozen version into the mutable version: > > >>> from dataclasses import dataclass > >>> @dataclass > ... class Example: > ... a: int > ... b: int > ... > >>> c = Example(1, 2) > >>> hash(c) > Traceback (most recent call last): > File "<stdin>", line 1, in <module> > TypeError: unhashable type: 'Example' > > >>> @dataclass(frozen=True) > ... class LockedExample(Example): > ... pass > ... > >>> Example.__hash__ = LockedExample.__hash__ > >>> hash(c) > 3713081631934410656 > > So "unsafe_hash=True" would just be a shorthand spelling of that which > skips creating the full frozen version of the class (and with the > explicit parameter, we can better document the risks of making > something hashable without also freezing it post-creation). > > Cheers, > Nick. > > -- > Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia > _______________________________________________ > 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/ > levkivskyi%40gmail.com >
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