On Sat, Mar 20, 2010 at 09:11, Antoine Pitrou <solipsis@pitrou.net> wrote:
Mark Dickinson <dickinsm <at> gmail.com> writes:
On Fri, Mar 19, 2010 at 9:50 PM, Guido van Rossum <guido <at> python.org>
wrote:
There is one choice which I'm not sure about. Should a mixed float/Decimal operation return a float or a Decimal?
I'll just say that it's much easier to return a Decimal if you want to be able to make guarantees about rounding behaviour, basically because floats can be converted losslessly to Decimals. I also like the fact that the decimal module offers more control (rounding mode, precision, flags, wider exponent range) than float.
A problem, though, is that decimals are much slower than floats. If you have a decimal creeping in some part of a calculation it could degrade performance quite a bit.
For a little context, we have this numeric tower: int -> Fraction -> float -> complex And coincidentally (or not), we have an unspoken rule that you can go right, but never left (int/int -> float, int/Fraction -> Fraction, Fraction/float -> float, Fraction/complex -> complex, etc). This gives us a preference for fast, inexact results. Decimal is more precise, and pays a performance cost for it. It also seems odd to stick it between float and complex (nobody's planning a ComplexDecimal, right?) That suggests it should go between Fraction and float. Decimal/float -> float. -- Adam Olsen, aka Rhamphoryncus