[Python-ideas] incremental hashing in __hash__

[Paul Moore]

On 5 January 2017 at 00:31, Steven D'Aprano <steve at pearwood.info> wrote:
> This is a good point. Until now, I've been assuming that
> hash.from_iterable should consider order. But frozenset shows us that
> sometimes the hash should *not* consider order.
>
> This hints that perhaps the hash.from_iterable() should have its own
> optional dunder method. Or maybe we need two functions: an ordered
> version and an unordered version.
>
> Hmmm... just tossing out a wild idea here... let's get rid of the dunder
> method part of your suggestion, and add new public class methods to
> tuple and frozenset:
>
>     tuple.hash_from_iter(iterable)
>     frozenset.hash_from_iter(iterable)
>
>
> That gets rid of all the objections about backwards compatibility, since
> these are new methods. They're not dunder names, so there are no
> objections to being used as part of the public API.
>
> A possible objection is the question, is this functionality *actually*
> important enough to bother?
>
> Another possible objection: are these methods part of the sequence/set
> API? If not, do they really belong on the tuple/frozenset? Maybe they
> belong elsewhere?

At this point I'd be inclined to say that a 3rd party hashing_utils
module would be a reasonable place to thrash out these design
decisions before committing to a permanent design in the stdlib. The
popularity of such a module would also give a level of indication as
to whether this is an important optimisation in practice.

Paul