Syllabus Lesson 70 of 141 · APIs nobody can misuse
APIs nobody can misuse

Money is not a float

Why it matters 0.1 + 0.2 is not 0.3 in binary floating point, and pounds plus euros is not a number at all. A money type that refuses both turns a class of silent accounting bugs into errors at the line that caused them.

A billing service passes amounts around as floats with a separate currency field. Two incidents this year: an invoice split three ways that came to a penny short, and a refund in euros added to a balance in pounds. You are writing the Money type the service will use instead.

Minor unit: the smallest coin of a currency: a penny for GBP, a cent for EUR and USD. JPY has none: amounts are whole yen.

Money("12.30", "GBP") + Money("0.70", "GBP")   # Money("13.00", "GBP")
Money("100.00", "GBP").split(3)                # 33.34, 33.33, 33.33
Your turn

Write class Money.

  • Money(amount, currency): amount is a str like "12.30", an int (whole units) or a decimal.Decimal; currency is "GBP", "EUR", "USD" (2 decimal places) or "JPY" (0).
  • Support + and - between Money, * by an int (either side), ==, <, <=, >, >= and str() ("GBP 12.30", "JPY 500").
  • split(n): n parts that add up exactly, larger first, differing by at most one minor unit.

Hidden misuse scripts will try these:

  • TypeError: a float or bool amount; Money plus or minus a non-Money; multiplying by anything but an int.
  • ValueError: an unknown currency (codes are upper case); an amount with more places than the currency has, or that is not a number; adding, subtracting or ordering two currencies; split(n) with n below 1.
  • Money equals only Money of the same currency and amount (anything else is simply not equal).

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