[Python-ideas] Python Numbers as Human Concept Decimal System

[Guido van Rossum]

On Fri, Mar 7, 2014 at 8:16 PM, Chris Angelico <rosuav at gmail.com> wrote:

> On Sat, Mar 8, 2014 at 3:11 PM, Guido van Rossum <guido at python.org> wrote:
> >> How is Decimal==float implemented? Is it by calling Decimal(rhs) and
> >> then comparing? If so, changing how Decimal(float) works won't break
> >> the invariant, as it'll make the same conversion each time.
>
> > There's a special function to convert the other operand for the purpose
> of
> > comparisons, and it currently uses Decimal.from_float(). I am not (yet
> :-)
> > proposing to change the behavior of that function -- it is and has always
> > been the API function that does exact float-to-Decimal conversion.
> > Decimal(repr(f)) == f returns False today (for the majority of float
> values
> > anyway) and under my proposal Decimal(f) == f would also return False. I
> > don't (yet :-) think that's a deal breaker.
>
> Maybe not a deal breaker, but certainly another panel on the bikeshed.
> It would make good sense to retain that invariant by making the
> comparison use repr just like the constructor.
>

Not sure I agree. I find the invariant Decimal(x) == x rather
uninteresting. I find the idea that Decimal.__eq__ compares the exact
mathematical values very appealing.

-- 
--Guido van Rossum (python.org/~guido)
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