[Tutor] Unexpected result from decimal

[Orri Ganel]

Jacob S. wrote:

> Hi all,
>
>    I'm having a problem that is ticking me off. (to put it lightly)
> Why does decimal do this --  I thought that getcontext().prec was 
> number of decimal places?
>
>>>> import decimal
>>>> decimal.getcontext().prec = 2
>>>> a = decimal.Decimal(2)
>>>> b = decimal.Decimal(3)
>>>> 100*a/b
>>>
> Decimal("67")
>
>>>> print 100*a/b
>>>
> 67
>
>>>>
>
> Why does it do this?
> It's really, really, messing things up for me because results are not 
> interpreted in my programs correctly.
> Jacob
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> http://mail.python.org/mailman/listinfo/tutor
>
Well, from looking at the documentation, decimal.getcontext().prec 
*tells* you the precision, so assigning to it is meaningless. Let's see 
what you have to do to actually change it:

 >>> a = decimal.Decimal(2)
 >>> b = decimal.Decimal(3)
 >>> 100*a/b
Decimal("66.66666666666666666666666667")
 >>> decimal.BasicContext
Context(prec=9, rounding=ROUND_HALF_UP, Emin=-999999999, Emax=999999999, 
capitals=1, flags=[], traps=[Clamped, DivisionByZero, InvalidOperation, 
Overflow, Underflow])
 >>> decimal.DefaultContext
Context(prec=28, rounding=ROUND_HALF_EVEN, Emin=-999999999, 
Emax=999999999, capitals=1, flags=[], traps=[DivisionByZero, 
InvalidOperation, Overflow])
 >>> decimal.ExtendedContext
Context(prec=9, rounding=ROUND_HALF_EVEN, Emin=-999999999, 
Emax=999999999, capitals=1, flags=[], traps=[])
 >>> decimal.setcontext(decimal.ExtendedContext)
 >>> decimal.getcontext().prec
9
 >>> decimal.setcontext(decimal.Context(prec=50, 
rounding=decimal.ROUND_HALF_EVEN, Emin=-999999999, Emax=999999999, 
capitals=1, flags=[], traps=[decimal.DivisionByZero, 
decimal.InvalidOperation, decimal.Overflow]))
 >>> decimal.getcontext().prec
50
 >>> 100*a/b
Decimal("66.666666666666666666666666666666666666666666666667")

Hold on a second . . . it looks like prec doesn't refer to how many 
digits there will be after the decimal point, but how many total digits 
. . . bit of an odd way to do things, but that explains your problem. 
Scratch what i said before about assigning to decimal.getcontext().prec, 
by the way, it looks like that works after all:

 >>> decimal.getcontext().prec = 4
 >>> 100 * a/b
Decimal("66.67")

HTH,
Orri

-- 
Email: singingxduck AT gmail DOT com
AIM: singingxduck
Programming Python for the fun of it.

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