When working with decimal numbers in Python, using floating-point literals as input to the Decimal constructor can lead to unintended imprecision due to the limitations of IEEE 754 Wikipedia 2025 floating-point representation; therefore, to ensure accurate decimal representation, it is advisable to avoid using floating-point literals.
In the noncompliant01.py code example, a floating-point value is given to the Decimal constructor. The decimal 0.45 cannot be exactly represented by a floating-point literal, and hence the output of the decimal is imprecise. This is because 0.45 as a floating-point representation deals with the number in binary. In binary, many decimal fractions cannot be represented exactly. As a result, 0.45 is stored as an approximate binary fraction in a float, and when this approximation is converted to a Decimal, the inexactness is preserved, can result in 0.450000000000000011102230246251565404236316680908203125.
"""Non-compliant Code Example"""
from decimal import Decimal
print(Decimal(0.45))
In the compliant01.py code example, the floating-point value is passed as a string, allowing the value to be directly converted to a Decimal object with the exact decimal value. This is because the string representation is interpreted exactly as it appears, maintaining all the specified digits.
"""Compliant Code Example"""
from decimal import Decimal
print(Decimal("0.45"))
| Tool | Version | Checker | Description |
|---|---|---|---|
| Bandit | 1.7.4 on python 3.10.4 | Not Available | |
| Flake8 | 8-4.0.1 on python 3.10.4 | Not Available |
| Wikipedia 2025 | IEEE 754 [online]. Available from: https://en.wikipedia.org/wiki/IEEE_754 |
| Python docs | decimal — Decimal fixed-point and floating-point arithmetic [online]. Available from: https://docs.python.org/3/library/decimal.html [accessed 2 February 2025] |