Recipes: Numbers and maths

Task-focused how-tos. If you know what you want to do but aren't sure how to build it, this is the section to skim.

Recipes in this section

• Round numbers correctly — why round(2.5) is 2, how to get the "round half up" most people expect, rounding to a number of decimal places or significant figures, and rounding money exactly.
• Handle money with Decimal — the end-to-end pattern for currency: build Decimals from strings, do exact arithmetic, quantize to two places with a chosen rounding rule, and never let a float near it.
• Format numbers for display — thousands separators, fixed decimal places, percentages, scientific notation, padding and alignment, signs, and other number formats with f-strings.
• Avoid common number mistakes — a catalogue of the traps: float equality, Decimal(0.1) versus Decimal('0.1'), mixing Decimal and float, nan comparisons, lossy sum, and more.