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

Nick Coghlan ncoghlan at gmail.com
Thu Mar 18 22:48:33 CET 2010

```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?

Cheers,
Nick.

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