Arithmetic and precision.
THE TWO MAIN TYPES
Whole numbers, and numbers with decimal points.
WHAT TO KNOW ABOUT DIVISION
Python offers ordinary division, which produces a decimal, and floor division, which discards the remainder.
Choose deliberately.
THE PRECISION PROBLEM
Decimal numbers are stored in a way that cannot represent some values exactly.
WHAT THAT CAUSES
Small unexpected differences in arithmetic.
WHY IT MATTERS
For money, that is unacceptable.
WHAT TO USE FOR MONEY
A decimal type designed for exact arithmetic, or work in the smallest unit as whole numbers.
Never ordinary decimals for financial calculations.
THAT POINT
The single most important numeric consideration for business code.
ROUNDING
Be explicit about how and when you round.
Rounding at different points produces different totals.
CONVERTING TEXT TO NUMBERS
Explicit, and it fails on unexpected input.
Handle that failure rather than assuming clean data.
WHAT TO CHECK
That values you treat as numbers actually are.
