[Python-ideas] Python Numbers as Human Concept Decimal System

[Stefan Krah]

Guido van Rossum <guido at python.org> wrote:
> On Mon, Mar 10, 2014 at 7:22 AM, Mark Dickinson <dickinsm at gmail.com> wrote:
>     On Mon, Mar 10, 2014 at 1:53 PM, Stefan Krah <stefan at bytereef.org> wrote:
> 
>         That is why I think we should seriously consider moving to IEEE
>         semantics
>         for a decimal literal.
> 
>     I think that would make a *lot* of sense.
> 
> What's the proposal? Just using decimal64 for decimal literals, or introducing
> decimal64 as a new builtin type? (I could get behind either one.)

IEEE 754-2008 is in a certain sense "arbitrary precision", since it allows
multiple discrete contexts: Decimal32, Decimal64, Decimal128, ...

In theory this goes on ad infinitum, but the precision increases much
slower than the exponents.  Mike Cowlishaw's spec fills in the gaps by
allowing arbitrary contexts.  Additionally there are some minor differences,
but if we make moderate changes to Decimal, we get strict IEEE behavior.


So my specific proposal was:

  1) Make those changes to Decimal (we can call the module decimal2
     if backwards compatibility rules it out). The most important
     change relevant to this subthread is rounding all *literals* on
     input while preserving exact construction via Decimal(value).

  2) Keep the arbitrary context facility.

  3) Set IEEEContext(Decimal64) as the default. Users will most
     likely primarily use Decimal32, Decimal64 and Decimal128.
     (Many users will likely never change the context at all.)

  4) Optional: Also use the function names from IEEE 754-2008.


With these changes most users would /think/ that Decimal is a
fixed width Decimal64 type.  Advanced users can still change the
context.  I know for a fact that some users really like the
option of increasing the precision temporarily.


I have to think about the other solution (decimal64 only). At first
glance it seems too restrictive, since I imagine users would at least
want Decimal128, too. Additionally there is no speed benefit.


Stefan Krah