On Jun 13, 12:56 am, geremy condra <debat...@gmail.com> wrote: > On Sat, Jun 12, 2010 at 4:40 PM, Robert Kern <robert.k...@gmail.com> wrote: > > On 2010-06-12 17:49 , geremy condra wrote: > > >> In Python3.2, calling math.erfc with a value in [-27.2, -30) raises > >> an OverflowError: math range error. This is inconsistent with the > >> erfc function from scipy (scipy.special.erfc) as well as with the C99 > >> function by the same name, both of which return 2. I suspect that > >> this is the result of the cutoff for the use of the continuing fraction > >> approximation of erfc beginning when abs(x)> 30, but I'm not sure. > >> Is this desired behavior or should I file a bug report? > > > Bug, I think. > > Bug filed,http://bugs.python.org/issue8986.
A bug indeed. Many thanks for catching and reporting this, Geremy; it would have been embarrassing for Python 2.7 to be released with this bug in it. (Well, I would have been embarrassed, anyway; not sure about anyone else.) In case anyone cares, the bug was due to a misinterpreted "errno" value: there's an "exp(-x*x)" term in the formulas for erf(x) and erfc(x), and that term underflows to zero at abs(x) = sqrt(1075*log(2)), which is around 27.297. Some system math libraries (unfortunately not including the ones I tested on; I *did* test, honest!) set errno on underflow to zero; this errno value was then being misinterpreted as an indication of overflow by Python's general libm wrappings. Anyway, it's fixed now. -- Mark -- http://mail.python.org/mailman/listinfo/python-list
