On Sat, Mar 20, 2010 at 4:58 AM, Mark Dickinson <dicki...@gmail.com> wrote: > On Sat, Mar 20, 2010 at 11:41 AM, Paul Moore <p.f.mo...@gmail.com> wrote: >> On 20 March 2010 04:20, Nick Coghlan <ncogh...@gmail.com> wrote: >>> In the case of floats and Decimals, there's no ambiguity here that >>> creates any temptation to guess - to determine a true/false result for a >>> comparison, floats can be converted explicitly to Decimals without any >>> loss of accuracy. For Fractions, the precedent has already been set by >>> allowing implicit (potentially lossy) conversion to binary floats - a >>> lossy conversion to Decimal wouldn't be any worse. >> >> Hmm, given that a float can be converted losslessly to a fraction, why >> was the decision taken to convert the fraction to a float rather than >> the other way round? > > I'm not sure of the actual reason for this decision, but one argument > I've seen used for other languages is that it's desirable for the > inexactness of the float type to be contagious: rationals are > perceived as exact, while floats are perceived as approximations.
You hit the nail on the head. > Note that this only applies to arithmetic operations: for > comparisons, an exact conversion *is* performed. This is much like > what currently happens with ints and floats in the core: a mixed-type > arithmetic operation between an int and a float first converts the int > to a float (possibly changing the value in the process). A mixed-type > comparison makes an exact comparison without doing such a conversion. > For example (in any non-ancient version of Python): > >>>> n = 2**53 + 1 >>>> x = 2.**53 >>>> n > x # compares exact values; no conversion performed > True >>>> n - x # converts n to a float before subtracting > 0.0 > >> I don't see a PEP for the fractions module, and my google-fu has >> failed to find anything. Was there a discussion on this? > > There's PEP 3141 (http://www.python.org/dev/peps/pep-3141/), which was > the motivation for adding the fractions module in the first place, and > there's the issue tracker item for the fractions module > (http://bugs.python.org/issue1682). -- --Guido van Rossum (python.org/~guido) _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
