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

[Terry Reedy]

On 3/18/2010 5:48 PM, Nick Coghlan wrote:
> Mark Dickinson wrote:
>> Could everyone live with making float<->Decimal comparisons raise an
>> exception in 2.7?
>
> I could, with the caveat that *if* this causes problems for real world
> code, then changing it to produce the correct answer (as per your patch)
> should be applied as a bug fix in both 2.7 and 3.2.
>
> Note that even in Py3k there are some fairly serious weirdnesses kicking
> around due to the intransitive nature of numeric equality though:
>
>>>> from decimal import Decimal as dec
>>>> set((1, 1.0, dec("1.0")))
> {1}
>>>> set((1.0, dec("1.0")))
> {1.0, Decimal('1.0')}
>
>>>> d = {}
>>>> from decimal import Decimal as dec
>>>> d[1] = d[1.0] = d[dec("1.0")] = 42
>>>> d
> {1: 42}
>>>> d[1.0] = d[dec("1.0")] = 42
>>>> d
> {1: 42}
>>>> del d[1]
>>>> d[1.0] = d[dec("1.0")] = 42
>>>> d
> {1.0: 42, Decimal('1.0'): 42}
>
> When there is a clear, correct way (based on Decimal.from_float) to make
> numeric comparison behave in accordance with the rules of mathematics,
> do we really want to preserve strange, unintuitive behaviour like the above?

I would still strongly prefer that equality intransitivity be fixed one 
way or another (I suggested two on the tracker). The many ways it makes 
sets misbehave was discussed in both of
http://mail.python.org/pipermail/python-list/2008-September/508859.html
http://bugs.python.org/issue4087

Terry Jan Reedy