Decimal place issues with floats and decimal.Decimal

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IEEE floating point is binary, not decimal. There is no fixed length binary fraction that is exactly 0.1, or any multiple thereof. It is a repeating fraction, like 1/3 in decimal.

Please read What Every Computer Scientist Should Know About Floating-Point Arithmetic

Other options besides a Decimal class are

  • using Common Lisp or Python 2.6 or another language with exact rationals

  • converting the doubles to close rationals using, e.g., frap

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