Python Decimal: parsing a decimal string differs from converting an existing float
Decimal preserves the value it receives; constructing it from a float does not recover the decimal text that originally produced that float.
- Applies when
- Numeric input processing that explicitly requires decimal representation
- Tested environment
- Python 3.14.7; exact-construction assertions passed on macOS 2026-11-07
Binary floating point cannot exactly represent many short decimal fractions. Decimal can represent decimal fractions directly, but the construction path matters.
Local assertions: Decimal('0.1') + Decimal('0.2') equaled Decimal('0.3'), while Decimal(0.1) did not equal Decimal('0.1'). The first case starts from exact decimal text. The second converts the already approximated binary value.
Illustrative input pipeline: when a file defines measurements as decimal strings and exact decimal interpretation is required, preserve those strings until Decimal parsing. Converting to float first and then wrapping the result defeats that goal. For numerical algorithms designed around binary floating point, converting every value to Decimal may instead add complexity without satisfying a useful requirement.
Choose representation, precision, rounding mode, and comparison rules from the application's numeric contract. Decimal is still finite-precision arithmetic: division such as one third can round, and a default context is not a domain specification.
Evidence: two local assertions plus official documentation, not validation of a financial, scientific, or regulatory calculation. Test conversion boundaries explicitly.
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