prePEP: Decimal data type

[Jp Calderone]

On Thu, Nov 06, 2003 at 03:37:52PM -0500, Aahz wrote:
> In article <vqlaif4imh9k98 at news.supernews.com>,
> John Roth <newsgroups at jhrothjr.com> wrote:
> >
> >AFAICS, there are only two solutions:
> >
> ><decimal>.add(<number>, [<result spec>])
> >
> >and
> >
> ><context>.add(<decimal>, <number>)
> 
> Both are too inconvenient for casual users.  I think that for the Money
> subclass, it'll probably make sense to require users to specify an
> explicit Context before the first operation (raising an exception if not
> initialized).  For Decimal, using the same defaults as Rexx will
> probably work just fine.

  That sounds like it would work, if you are forced to specify the context
on a per-instance basis.  Otherwise, I don't see how two different modules
which wanted to treat Money differently could ever be used together.

  How about this instead?

    import context

    class Decimal:
        ROUNDING_STYLE = 'decimal-rounding-style'
        ITALIAN, FRENCH, AMERICAN = range(3)

        def __add__(self, other):
            rstyle = context.get(Decimal.ROUNDING_STYLE, 'sane default')
            # Do stuff, depending on rstyle
            return stuff


    def jiggerSomeMoneys(a, b, c):
        # Lots of operations on a, b, and c

    context.call({Decimal.ROUNDING_STYLE: Decimal.ITALIAN}, jiggerSomeMoneys, a, b, c)


  This way, only the top-most invocation need deal with the context.  The
actual implementation dealing with decimals is free to ignore it completely,
and should it require a change, there is only a single place where that
change is required.

  As an added bonus, the context code is completely independent of the
decimal code, and can be re-used for any other application requiring
contextual data.

  Implementations of context.call and context.get are left as an exercise to
the reader (Eh, ok, I can show you mine if anyone is actually interested :)

  Jp