[Python-Dev] Re: Decimal type question [Prothon]

[Tim Peters]

[Mark Hahn]
> Forgive me if I'm being a pest,

Nope, not at all.

> but no one has commented on the real reason I asked the question.  What
> does everyone think of the idea of having these three built-in numeric types?

It depends on your goals for Prothon, and also on the relative
performance of the various numeric implementations.  Semantically, it
would be nicest if Python used the IBM decimal type for everything by
default (whether spelled 123, 123.0, or 123e0).  That's not gonna
happen, both because of backward compatibility, and because our
implementation of that type is "too slow".

I don't know how fast or slow the .NET Decimal type is on its own, let
alone compared to whatever implementations of bigints and binary
floats you're using.  Speed may or may not matter to your target
audience (although, in reality, you'll have multiple audiences, each
with their own irrational demands <wink>).

> 1) An Int implemented with infinite precision integer (Python Longs) with
> the constant/__str__ form of +-NN such as 0, -123, 173_394, etc.
>
> 2) A Decimal implemented with the .Net decimal float (or the IBM decimal if
> the .Net decimal sucks too much) with the constant/__str__ form of +-NN.NN
> such as 0.0, -123.0, 173_394.0, 173394.912786765, etc.
>
> 3) A binary Float implemented with the hardware floating point with the
> constant/__str__ form of +-NN.NN+-eNN such as 0e0, -123e0, 173.39e+3,
> 2.35e-78, etc.

Sounds decent to me, although the IBM flavor of decimal is a full
floating-point type, and some apps will definitely want to use it for
6.02e23 thingies instead of the native binary Float.  Maybe use "d" as
a "decimal float" exponent marker.

> There would be no automatic conversion except the / operator would convert
> from Int to Decimal and the Math module would convert Int and Decimal
> values to Float for almost all functions (except simple ones like abs, min,
> max).

Int * Decimal will be very common in financial apps, and the latter
are a prime use case for Decimal ("OK, they bought 200 copies of
'Programming Prothon' at 39.98 each", etc).  Numeric auto-conversions
that don't lose information are usually helpful.