[Python-ideas] incremental hashing in __hash__

[Nick Coghlan]

On 29 December 2016 at 18:35, Chris Angelico <rosuav at gmail.com> wrote:

> On Thu, Dec 29, 2016 at 7:20 PM, Steven D'Aprano <steve at pearwood.info>
> wrote:
> > I'd rather add a generator to the itertools
> > module:
> >
> >     itertools.iterhash(iterable)  # yield incremental hashes
> >
> > or, copying the API of itertools.chain, add a method to hash:
> >
> >     hash.from_iterable(iterable)  # return hash calculated incrementally
>
> The itertools module is mainly designed to be consumed lazily. The
> hash has to be calculated eagerly, so it's not really a good fit for
> itertools.


I understood the "iterhash" suggestion to be akin to itertools.accumulate:

    >>> for value, tally in enumerate(accumulate(range(10))): print(value,
tally)
    ...
    0 0
    1 1
    2 3
    3 6
    4 10
    5 15
    6 21
    7 28
    8 36
    9 45

However, I think including Ned's recipe (or something like it) in
https://docs.python.org/3/reference/datamodel.html#object.__hash__ as a
tool for avoiding large temporary tuple allocations may be a better way to
start off as:

1. It's applicable to all currently released versions of Python, not just
to 3.7+
2. It provides more scope for people to experiment with their own variants
of the idea before committing to a *particular* version somewhere in the
standard library

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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