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

Mark Dickinson dickinsm at gmail.com
Wed Mar 17 14:39:14 CET 2010

On Tue, Mar 16, 2010 at 10:16 PM, Greg Ewing
<greg.ewing at canterbury.ac.nz> wrote:
> Mark Dickinson wrote:
>> On Tue, Mar 16, 2010 at 3:58 PM, P.J. Eby <pje at telecommunity.com> wrote:
>>> If not, it might be confusing if a number that prints as '.1' compares
>>> unequal to Decimal('.1').
>> Agreed, but this is just your everyday floating-point confusion, to be
>> dealt with by social means (e.g., educating the programmer).
> Seems to me that this education would mostly consist of saying
> "don't compare floats and decimals", which is why I think that
> disallowing them in the first place would be better.

I was thinking of something more along the lines of:  "Sure, go ahead
and compare floats and decimals, but be aware that float('1.1') is not
exactly 1.1, so don't complain when 1.1 == Decimal('1.1') returns
False."  For me, this actually a plus of allowing these comparisons:
it makes the education easier.  "Look, the binary float stored for 1.1
is actually larger than 1.1, and here's the proof:  >>> 1.1 >
Decimal('1.1')   -> True."


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