[Python-ideas] incremental hashing in __hash__

[Neil Girdhar]

On Thu, Jan 5, 2017 at 9:10 PM Stephen J. Turnbull <
turnbull.stephen.fw at u.tsukuba.ac.jp> wrote:

> Paul Moore writes:
>
>  > The debate here regarding tuple/frozenset indicates that there may not
>  > be a "standard way" of hashing an iterable (should order matter?).
>
> If part of the data structure, yes, if an implementation accident, no.
>
>  > Although I agree that assuming order matters is a reasonable
>  > assumption to make in the absence of any better information.
>
> I don't think so.  Eg, with dicts now ordered by insertion, an
> order-dependent default hash for collections means
>
>     a = {}
>     b = {}
>     a['1'] = 1
>     a['2'] = 2
>     b['2'] = 2
>     b['1'] = 1
>     hash(a) != hash(b)    # modulo usual probability of collision
>
> (and modulo normally not hashing mutables).  For the same reason I
> expect I'd disagree with Neil's proposal for an ImmutableWhatever
> default __hash__ although the hash comparison is "cheap", it's still a
> pessimization.  Haven't thought that through, though.
>

I don't understand this?  How is providing a default method in an abstract
base class a pessimization?  If it happens to be slower than the code in
the current methods, it can still be overridden.


>
> BTW, it occurs to me that now that dictionaries are versioned, in some
> cases it *may* make sense to hash dictionaries even though they are
> mutable, although the "hash" would need to somehow account for the
> version changing.  Seems messy but maybe someone has an idea?
>

I think it's important to keep in mind that dictionaries are not versioned
in Python. They happen to be versioned in CPython as an unexposed
implementation detail.  I don't think that such details should have any
bearing on potential changes to Python.


> Steve
>
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