[Tutor] decimal precision in python

[Steve Willoughby]

On 07-Feb-12 03:15, Steven D'Aprano wrote:
> Steve Willoughby wrote:
>> If you need lots of precision, you might consider using the decimal
>> class. It'll cost you speed vs. the native floating-point type but
>> won't cause you round-off errors.
>
> I'm afraid that's not correct. Decimal is still subject to rounding errors.
>
>  >>> from decimal import Decimal
>  >>> x = 1/Decimal(3) # one third, as close as a Decimal can give
>  >>> x + x + x == 1
> False

Sorry, I guess I took it for granted that was understood.  I was 
referring to round-off caused by binary representation of a number that 
by all appearances is a simple, rational, non-repeating decimal number 
expressed in base 10.

In other words, when you're adding up financial figures, you'll get an 
accurate sum without "mysterious" rounding off, but of course certain 
fractions which don't have a straightforward way to represent as a 
decimal value, like 1/3, are going to be an issue.  Good catch, though, 
it was better to point that out.


-- 
Steve Willoughby / steve at alchemy.com
"A ship in harbor is safe, but that is not what ships are built for."
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