Richard D. Moores wrote:
Python 3.1
The decimal module continues to puzzle me from time to time. Here's
one of those. I want to use Alex Martelli's factory function as much
as possible. Turns out it has a problem with precision in addition and
multiplication.
=========================================
from decimal import Decimal as D
import decimal
#Alex Martelli's factory function from 'Python in a Nutshell', 2nd ed., p.373
def d(x):
return decimal.Decimal(str(x))
I don't have the second edition of the book, only the first. Are you
sure that Alex Martelli calls this a *factory* function? It doesn't look
like a factory to me.
decimal.getcontext().prec = 55
print('power')
a = D('123.2345274523452345235432452345')**D('2.3')
a2 = d(123.2345274523452345235432452345)**d(2.3)
print('a =', a)
print('a2 =', a2)
print()
And here is your problem:
>>> str(123.2345274523452345235432452345)
'123.234527452'
Consequently:
>>> D(str(123.2345274523452345235432452345))
Decimal('123.234527452')
Why in the world does precision not work for addition and
multiplication (see x2 and z2)?
It does. You are misinterpreting the error. The problem is not with the
addition, but with the construction *before* the addition.
By the way, if you are using Python 2.7 or 3.1, you should consider
using the Decimal.from_float method.
--
Steven
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