[Python-ideas] Specific equality method for hashable objects

[Brett Cannon]

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 <cpcloud at 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.
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