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

[Case Vanhorsen]

On Sat, Mar 20, 2010 at 10:05 AM, Mark Dickinson <dickinsm at gmail.com> wrote:
> On Sat, Mar 20, 2010 at 3:17 PM, Case Vanhorsen <casevh at gmail.com> wrote:
>> On Sat, Mar 20, 2010 at 4:06 AM, Mark Dickinson <dickinsm at gmail.com> wrote:
>>> 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.
>
> Does gmpy only do this for Python 2.6?  Or does it use different
> algorithms for 2.4/2.5 and 2.6?  As far as I can tell, there was no
> reasonable way to compute long_hash directly at all before the
> algorithm was changed for 2.6, unless you imitate exactly what Python
> was doing (break up into 15-bit pieces, and do all the rotation and
> addition exactly the same way), in which case you might as well be
> calling long_hash directly.
>
> Mark
>

It does the later: it converts from GMP's internal format to CPython's
long format and calculates the hash along the way. I may (should :) )
revert back to converting to long and then calling long_hash. The
majority of the speed increase came from the conversion improvements,
not the hash calculation.

I am in favor of any change that makes 2.7 and 3.2 behave the same.

casevh