[Python-Dev] calculator module

[Andrew P. Lentvorski, Jr.]

On Wed, 10 Mar 2004, Raymond Hettinger wrote:

> To these, the HP12C financial calculator adds:
> * solve for missing variable in (n i% pv pmt fv)
> * internal rate of return and net present value for a stream of cash
> flows
...
> By "pretty good" implementation, I mean that the implementations should
> remain as straight-forward as possible and neither promise, nor get in a
> snit about being accurate to the last place (i.e. a 9 place
> approximation of gamma would be considered fine).

Pretty good is not "good enough".  One of the reasons the HP 12C is so
popular is that every 12C gets *exactly* the same answer for the same
inputs regardless of age, model, etc.  The Python version would have to be
a bit exact match to a 12C for that calculation.

In addition, there are going to be requirements for directed rounding to
ensure the stability of some functions over their domains.  Directed
rounding generally causes a machine wide pipeline drain and lots of
ancillary machine wide FP effects.  Having Python cause FP random FP bugs
in other programs would not be good for its reputation.

Personally, I would rather see extremely good and accurate, but possibly
complex versions of these mathematical functions added into Python proper
with the calculator module simply being a wrapper around the base library
functions.  That way, the calculator automatically benefits when the
"state of the art" method in the main library advances or is corrected.

-a