Decimal Module in Python

Question

Why does this code output 25 although I used the decimal module. I tried it on the calculator and it outputted 26.28.

from decimal import *
getcontext().prec = 2

targetPrice = 40
currentPrice = 56.5
winChance = 0.9
stopLoss = 23
lossChance = 0.1

expectancy =100 * abs(((Decimal(targetPrice) -  Decimal(currentPrice)) / Decimal( currentPrice))) *  Decimal(winChance)
print(expectancy)

You are doing all of your calculations with a precision of 2. This introduces a fair amount of round-off error. Your calculator uses more like 15 decimal places of precision. Not surprisingly, it is more accurate. The decimal module is intended for calculations which are higher precision than a 64-bit float. For some reason you are using it for lower-precision calculations. — John Coleman
– John Coleman, Commented Oct 6, 2021 at 16:44
Applying Decimal() to floating-point values entirely misses the point of the Decimal module. 0.9, for example, is not a value that can be accurately represented as a float; turning it into a Decimal results in a precise value that isn't quite equal to 0.9. You have to use strings - Decimal('0.9') for example - to produce exact values. — jasonharper
– jasonharper, Commented Oct 6, 2021 at 16:47

Frank Yellin · Accepted Answer · 2021-10-06 16:45:52Z

2

You're getting inexact results because you're rounding down to two decimal places after every calculation. Your calculator isn't doing that.

With no rounding.

>>> (Decimal(40) - Decimal(56.5)) / Decimal(56.5)
Decimal('-0.2920353982300884955752212389')

With rounding:

>>> (Decimal(40) - Decimal(56.5)) / Decimal(56.5)
Decimal('-0.28')

It only gets worse after that.

answered Oct 6, 2021 at 16:45

Frank Yellin

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1 Comment

Jérôme Over a year ago

Should we use strings as suggested in @jasonharper's comment under OP?

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