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

[Oscar Benjamin]

On 8 March 2014 00:10, Guido van Rossum <guido at python.org> wrote:
> On Fri, Mar 7, 2014 at 3:42 PM, Oscar Benjamin <oscar.j.benjamin at gmail.com>
> wrote:
>>
>> The current Decimal constructor comes with a guarantee of exactness.
>
> But Decimal(<float>) is relatively new (it was an error in 2.6). I know it's
> annoying to change it again, but I also have the feeling that there's not
> much use in producing a Decimal with 53 (or more?) *decimal digits* of
> precision from a float with 53 *bits* -- at least not by default. Maybe we
> can relent on this guarantee and make the constructor by default use fewer
> digits (e.g. using the same algorithm used by repr(<float>)) and have an
> optional argument to produce the full precision? Or reserve
> Decimal.from_float() for that?

I'd rather a TypeError than an inexact conversion. The current
behaviour allows to control how rounding occurs. (I don't use the
decimal module unless this kind of thing is important).

>> It accepts int, str and float (and tuple) and creates a Decimal object
>> having the exact value of the int or float or the exact value that
>> results from a decimal interpretation of the str. This exactness is
>> not mandated by any of the standards on which the decimal module is
>> based but I think it is a good thing. I strongly oppose a change that
>> would have it use heuristics for interpreting other numeric types.
>
> Well, given that most floating point values are the result of operations
> that inherently rounded to 53 bits anyway, I'm not sure that this guarantee
> is useful. Sure, I agree that there should be *a* way to construct the exact
> Decimal for any IEEE 754 double (since it is always possible, 10 being a
> multiple of 2), but I'm not sure how important it is that this is the
> default constructor.

It's hard to reason about the accuracy of calculations that use the
decimal module. It's harder than with a fixed-width type because of
the possible presence of higher-than-context precision Decimals. The
invariant that the constructor is exact (or a TypeError) is of
significant benefit when using Decimal for code that accepts inputs of
different numeric types.


Oscar