This goes against "there's only one way to do it" and overriding equality and hashing is niche enough that I don't think it warrants having two ways of doing it (because you can't drop overriding __eq__/__hash__ due to backwards-compatibility concerns).
On Wed, May 8, 2019 at 9:50 AM Phillip Cloud <cpcl...@gmail.com> wrote: > A number of libraries override the equality operator (__eq__) in such a > way that it returns an object that isn't a Python bool. This all but > precludes making these objects hashable, even though there's nothing in > principle that prevents them from being so. > > The main use case I have in mind is an expression DSL that returns another > expression when its __eq__ method is called (sqlalchemy is a good example > of such a DSL). In such DSLs you often want to use expressions as keys in > dict. You *can* use them as keys, but the code that does is only reliable > insofar as it can guarantee that the expressions in the dict are identical > (in the sense of a pointer) to the values you're going to later use for > lookup. > > After thinking about this for a bit, I'd like to understand whether it's > reasonable to propose a new method __hash_eq__ that would be used during > dict key comparisons (in lookdict). > > This new method would allow an object to override __eq__ *and* be > hashable, by defining __hash_eq__. __hash_eq__ would only be allowed to > return a bool, unlike __eq__. > > Regarding backwards compatibility, __hash_eq__ would be defined by default > to be equivalent to __eq__. > > I haven't thought about other standard library mappings like OrderedDict > yet, and it's entirely possible that such considerations would kill this > idea. > > I did some googling before writing this up, and I couldn't find anything > that proposed anything like this but perhaps my google skills are lacking. > If there's a history here that I'm missing I'm happy to learn more about it. > _______________________________________________ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ >
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