float limits

[Terry Reedy]

"Alexandre Fayolle" <alf at leo.logilab.fr> wrote in message
 > Is there a way of knowing what the value of the smallest positive
float
> available on a given machine from within python? A bit like
sys.maxint,
> but for mindouble. Similarly, I'd like to know the value of the
biggest
> positive float, maxfloat. If the solution is portable, that's all
the better.

There is a standard algorithm that numerical analysts use to calculate
machine epsilon:
I believe its something like

eps1 = 1.0
eps2 = 0.5
while (1.0+eps2 != 1.0):
   eps1 = eps2
   eps2 /= 2.0

I think adding 0.0 instead of 1.0 and comparing might be theoretically
just as good, but good compilers will optimize away addition.  Simply
testing eps2 against 0.0 fails if min/2 = min instead of 0.

Terry J. Reedy