[Python-Dev] Adventures with Decimal

[Greg Ewing]

Raymond Hettinger wrote:

> IMO, user input (or
> the full numeric strings in a text data file) is sacred and presumably
> done for a reason -- the explicitly requested digits should not be
> throw-away without good reason.

I still don't understand what's so special about the
input phase that it should be treated sacredly, while
happily desecrating the result of any *other* operation.

To my mind, if you were really serious about treating
precision as sacred, the result of every operation
would be the greater of the precisions of the
inputs. That's what happens in C or Fortran - you
add two floats and you get a float; you add a float
and a double and you get a double; etc.

> Truncating/rounding a
> literal at creation time doesn't work well when you are going to be
> using those values several times, each with a different precision.

This won't be a problem if you recreate the values
from strings each time. You're going to have to be
careful anyway, e.g. if you calculate some constants,
such as degreesToRadians = pi/180, you'll have to
make sure that you recalculate them with the desired
precision before rerunning the algorithm.

> Remember, the design documents for the spec state a general principle:
> the digits of a decimal value are *not* significands, rather they are
> exact and all arithmetic on the is exact with the *result* being subject
> to optional rounding.

I don't see how this is relevant, because digits in
a character string are not "digits of a decimal value"
according to what we are meaning by "decimal value"
(i.e. an instance of Decimal). In other words, this
principle only applies *after* we have constructed a
Decimal instance.

-- 
Greg Ewing, Computer Science Dept, +--------------------------------------+
University of Canterbury,	   | A citizen of NewZealandCorp, a	  |
Christchurch, New Zealand	   | wholly-owned subsidiary of USA Inc.  |
greg.ewing at canterbury.ac.nz	   +--------------------------------------+