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>>> from decimal import Decimal
>>> Decimal('0.3637') * (Decimal('1') / Decimal('0.9323')) == Decimal('0.3637') / Decimal('0.9323')
False

Why?

I thought decimal module solves problems with rounding in floating point arithmetic. How to check the equality of two decimal numbers in Python?

jpp
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Aleksandr Gavrilov
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    Cannot reproduce, returns `True` for me (once I fix the errant `)`) – Cory Kramer Mar 22 '18 at 11:26
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    Because `(Decimal('1')/Decimal('0.9323')) = Decimal('1.072616110693982623619006757')` . And hence `Decimal('0.3637') * Decimal('1.072616110693982623619006757') != Decimal('0.3637')/Decimal('0.9323')`. – the.salman.a Mar 22 '18 at 11:31

1 Answers1

1

Observation

Order of operations is critical to understand why you are seeing the result:

from decimal import Decimal as D

D('0.3637') * (D('1') / D('0.9323')) == D('0.3637') / D('0.9323')
# False

(D('0.3637') * D('1')) / D('0.9323') == D('0.3637') / D('0.9323')
# True

Explanation

The reason is given here, excerpt below. Decimal arithmetic is still fundamentally finite precision.

You can and should, if it matters to you, reduce precision using getcontext().prec. See decimal documentation for more details.

In general decimal is probably going overboard and still will have rounding errors in rare cases when the number does not have a finite decimal representation (for example any fraction where the denominator is not 1 or divisible by 2 or 5 - the factors of the decimal base (10)).

Solution

For general comparison of floats:

What is the best way to compare floats for almost-equality in Python?

jpp
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