[D66] The Illusion of Exactness: Rounding Errors in Python’s Decimal Module

[René Oudeweg]

[paper]

https://drive.google.com/file/d/1Pmyt1m4J7HUVljPjYeH_HLKm1gWadx_S/view?usp=sharing


The Illusion of Exactness:
Rounding Errors in Python’s Decimal Module
Anonymous

January 3, 2026

Abstract

Python’s decimal module is often presented as a solution to the 
floating-point representation problems inherent in binary arithmetic. 
However, this belief in decimal exactness obscures a deeper truth: 
Decimal merely shifts the locus of rounding error rather than elim-
inating it. This paper examines the sources of rounding in decimal 
arithmetic, demonstrates concrete examples of decimal rounding errors, 
and explores the philosophical implications of finite-precision 
arithmetic in any base system.