[Python-Dev] Mixing float and Decimal -- thread reboot

Stefan Krah stefan at bytereef.org
Mon Mar 22 20:46:28 CET 2010

Raymond Hettinger <raymond.hettinger at gmail.com> wrote:
> On Mar 22, 2010, at 11:26 AM, Mark Dickinson wrote:
> > 
> > Just for the record, I'd also prefer Decimal + Fraction -> Decimal.
> Guido was persuasive on why float + Fraction --> float,
> so this makes sense for the same reasons.
> For the implementation, is there a way to avoid the double rounding
> in   myfloat + myfrac.numerator / myfrac.denominator?
> Perhaps translate it to:
>       f = Fractions.from_decimal(myfloat) + myfract   # Lossless, exact addition
>       return f.numerator / f.denominator           # Only one decimal context rounding applied.
> Elsewhere in the decimal module, there is a fundamental notion
> that numbers are exact and only the results of operations are
> rounded.   For example, it is possible in decimal to add together
> two high precision numbers but do so in a low precision context.

I don't think this will be practical for huge decimal exponents.

Also, at first glance I wonder how to integrate this cleanly into the
control flow. convert_other() will not work, so there would need to
be a special case for fractions in each function.

Or do you mean to outsource the whole computation to the fractions
module, which calls decimal only for the final division?

Stefan Krah

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