[Python-Dev] Decimal <-> float comparisons in py3k.

[Case Vanhorsen]

On Sat, Mar 20, 2010 at 4:06 AM, Mark Dickinson <dickinsm at gmail.com> wrote:
> On Fri, Mar 19, 2010 at 1:17 PM, Case Vanhorsen <casevh at gmail.com> wrote:
>> On Fri, Mar 19, 2010 at 3:07 AM, Mark Dickinson <dickinsm at gmail.com> wrote:
>>> On Fri, Mar 19, 2010 at 9:37 AM, Mark Dickinson <dickinsm at gmail.com> wrote:
>>>> Making hashes of int,
>>>> float, Decimal *and* Fraction all compatible with one another,
>>>> efficient for ints and floats, and not grossly inefficient for
>>>> Fractions and Decimals, is kinda hairy, though I have a couple of
>>>> ideas of how it could be done.
>>>
>>> To elaborate, here's a cute scheme for the above, which is actually
>>> remarkably close to what Python already does for ints and floats, and
>>> which doesn't require any of the numeric types to figure out whether
>>> it's exactly equal to an instance of some other numeric type.
>
>> Will this change the result of hashing a long? I know that both gmpy
>> and SAGE use their own hash implementations for performance reasons. I
>> understand that CPython's hash function is an implementation detail,
>> but there are external modules that rely on the existing hash
>> behavior.
>
> Yes, it would change the hash of a long.
>
> What external modules are there that rely on existing hash behaviour?

I'm only aware of  gmpy and SAGE.

> And exactly what behaviour do they rely on?

Instead of calculating hash(long(mpz)), they calculate hash(mpz)
directly. It avoids creation of a temporary object that could be quite
large and is faster than the two-step process. I would need to modify
the code so that it continues to produce the same result.

casevh
> Mark
>