Setting longer default decimal precision

[Steven D'Aprano]

On Mon, 18 Nov 2013 14:14:33 +0000, Kay Y. Jheallee wrote:

> Using 1/3 as an example,

[snip examples]

> which seems to mean real (at least default) decimal precision is limited
> to "double", 16 digit precision (with rounding error).

That's because Python floats actually are implemented as C doubles. And 
no, they're not configurable.

However, Python also has a Decimal class, which (unlike floats) are 
actually decimal rather than binary, and include configurable precision. 
There is a performance hit -- prior to Python version 3.3, Decimal was 
quite slow, but in 3.3 they got a major speed increase and are now nearly 
as fast as floats.

An example:

py> import decimal
py> x = decimal.Decimal(1)/3
py> decimal.getcontext().prec = 50
py> y = decimal.Decimal(1)/3
py> print(x, y)
0.3333333333333333333333333333 
0.33333333333333333333333333333333333333333333333333



-- 
Steven