The Ledger That Leaked Cents — Python Bug Hunt

Inspired by every audit that ever found the books off by a few cents — because prices took a round-trip through binary floating point.

Language: Python
Layer: Database
Difficulty: Easy
Concepts: Floating Point, Money
Modelled on: Floating-point money lore
Visible tests: the classic 19.99 is exact; sub-dollar prices are exact; whole and single-decimal prices pad
Reward: 50 XP for a complete fix

Briefing

Inspired by every audit that ever found the books off by a few cents — because prices took a round-trip through binary floating point. 19.99 * 100 is 1998.9999…, and int() finishes the crime.

prices.py parses price strings into integer cents. No floats allowed anywhere.

Bug report

BUG-1CENT · Priority: High · Reported by: finance close

parse_cents(price) — strings with exactly 0–2 decimals:

"19.99" -> 1999
"0.29" -> 29
"5" -> 500
"7.5" -> 750

Observed: "19.99" parses to 1998. Every 19.99 sale leaks a cent.

Logs

[close] ledger drift this quarter: -$142.17

The code as shipped

src/fin/prices.py (editable)

# Parses a price string into integer cents.

def parse_cents(price):
    return int(float(price) * 100)

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