[Python-Dev] Mixing float and Decimal -- thread reboot (original) (raw)

Greg Ewing greg.ewing at canterbury.ac.nz
Tue Mar 23 00:05:47 CET 2010

Mark Dickinson wrote:

But the Fraction type is going to mess this up: for Decimal + Fraction -> Decimal, I don't see any other sensible option than to convert the Fraction using the current context, since lossless conversion isn't generally possible.

You could convert the Decimal to a Fraction, do the arithmetic as Fractions, and convert the result back to Decimal using the current precision.

I think this would satisfy the principle of performing the calculation as though infinite precision were available, and the final rounding is justified, since seen from the outside we are performing a single operation on two numbers.

Another approach would be to convert the numerators and denominators separately to Decimal, and then do

a/b + c/d = (ad + bc) / (b*d)

using the usual rules of Decimal arithmetic. This would incur more than one rounding, but the rules for Decimals concern operations between two Decimals, and we have a Decimal and a Fraction here, so all bets could be considered off.

-- Greg

More information about the Python-Dev mailing list