Rounded Out of a Refund — Python Bug Hunt

Briefing

Inspired by the invoicing disputes born from Python's own round() — it's banker's rounding: round(2.5) is 2, round(3.5) is 4. Finance signed off on classic half-up ("half away from zero"). The refunds engine uses the built-in.

rounding.py — implement true half-up.

Bug report

BUG-HALFUP · Priority: High · Reported by: billing disputes

half_up(x) — round half AWAY from zero:

• 2.5 -> 3, 3.5 -> 4, 2.4 -> 2
• -2.5 -> -3

Observed: round(2.5) == 2 shaved cents off exactly-half refunds; the ombudsman letter cites 40,000 affected invoices.

Logs

[refund] 2.5 -> 2 (customer expected 3)

The code as shipped

src/fin/rounding.py (editable)

# Rounds to the nearest whole unit, halves away from zero.

def half_up(x):
    return round(x)