[Tutor] Precision with Decimal

[Kent Johnson]

On Sat, Jan 31, 2009 at 10:05 PM,  <gslindstrom at gmail.com> wrote:
> I am using the decimal module to work with money (US dollars and cents) and
> do not understand the precision. The documentation states:
>
> "The decimal module incorporates a notion of significant places so that 1.30
> + 1.20 is 2.50. The trailing zero is kept to indicate significance. This is
> the customary presentation for monetary applications."
>
> But I get:
>>>> from decimal import Decimal
>>>> a = Decimal('1.25')
>>>> a
> Decimal('1.25')
>>>> b = Decimal('2.50')
>>>> b
> Decimal('2.50')
>>>> a+b
> Decimal('3.8')
>
> I expect (and would like) a+b to be '3.75'. I've read through the
> getcontext() section but must be missing something. Can you help?

I get a different result, are you sure you didn't set the precision
before you did the above?
In [31]: from decimal import *
In [32]: a=Decimal('1.25')
In [33]: b=Decimal('2.50')
In [34]: a+b
Out[34]: Decimal('3.75')

The precision is the number of significant digits, not the number of
decimal places:
In [37]: getcontext().prec=1

In [38]: a+b
Out[38]: Decimal('4')

In [39]: getcontext().prec=2

In [40]: a+b
Out[40]: Decimal('3.8')

In [41]: a*Decimal('11111')
Out[41]: Decimal('1.4E+4')

The example is the docs is perhaps not the best one because 1.3 + 1.2
works correctly with normal floating point. 1.1 + 1.1 gives a
different result with Decimal vs floating point:
In [46]: 1.1 + 1.1
Out[46]: 2.2000000000000002

In [48]: Decimal('1.1') + Decimal('1.1')
Out[48]: Decimal('2.2')


Kent