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From: Oscar Benjamin <oscar.j.benjamin@gmail.com>
Date: Thu, 27 Feb 2014 23:50:55 +0000
Subject: Re: extend methods of decimal module
To: "Mark H. Harris" <harrismh777@gmail.com>
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Xref: csiph.com comp.lang.python:67179

On 27 February 2014 23:00, Mark H. Harris <harrismh777@gmail.com> wrote:
> On Thursday, February 27, 2014 10:24:23 AM UTC-6, Oscar Benjamin wrote:
>
>>>>> from decimal import Decimal as D
>> >>> D(0.1)
>> Decimal('0.1000000000000000055511151231257827021181583404541015625')
>
> hi Oscar,  well,  that's not what I'm doing with my  D()...  I'm not just making D() mimic Decimal... look inside it...  there's a  str()  call....   consider the following experiment and you'll see what I'm talking about...

I understood what your code is doing but I'm not sure if you do.
Calling str on a float performs an inexact binary to decimal
conversion. Calling Decimal on a float performs an exact binary to
decimal conversion. Your reasoning essentially assumes that every
float should be interpreted as an approximate representation for a
nearby decimal value. This is probably true if the user wrote "a =
0.1" but is generally not true in the kind of numeric code that is
likely to be using the transcendental functions defined in your dmath
module.

Calling Decimal(str(float)) introduces entirely avoidable inaccuracy
in your code when the primary purpose of your code as accuracy!


Oscar