Fix the money and the window

Shell (bash)

%%bash
base=https://academy.cloudtechanalytics.com/datasets/refunds
for f in refunds.py db.py app.py schema.sql customers.csv orders.csv order_items.csv test_refunds.py pytest.ini; do
  curl -sO "$base/$f"
done
cp refunds.py refunds_original.py
cat > refunds.py <<'EOF'
"""Refund rules for Kasuwa. Money is in whole kobo (integers)."""
from decimal import ROUND_HALF_UP, Decimal

RETURN_WINDOW_DAYS = 14
RESTOCKING_FEE = Decimal("0.10")  # charged on 'changed_mind' returns
REASONS = {"failed_delivery", "damaged", "wrong_item", "changed_mind"}


def round_kobo(amount):
    """Round a Decimal amount of kobo to a whole kobo, halves up."""
    return int(amount.quantize(Decimal("1"), rounding=ROUND_HALF_UP))


def item_refund(quantity, unit_price_kobo, reason):
    """Refund for one order line, in kobo."""
    amount = Decimal(quantity * unit_price_kobo)
    if reason == "changed_mind":
        amount *= 1 - RESTOCKING_FEE
    return round_kobo(amount)


def within_window(delivered_on, today):
    """True if a return is allowed: on or before day 14 after delivery."""
    return (today - delivered_on).days <= RETURN_WINDOW_DAYS


def refund_amount(items, reason, delivery_fee_kobo):
    """Total refund in kobo. items is a list of (quantity, unit_price_kobo) pairs."""
    total = sum(item_refund(quantity, price, reason) for quantity, price in items)
    if reason in ("failed_delivery", "damaged", "wrong_item"):
        total += delivery_fee_kobo
    return total
EOF
diff -u refunds_original.py refunds.py | tail -n +3

@@ -1,22 +1,27 @@
-"""Refund rules for Kasuwa."""
-from datetime import date
+"""Refund rules for Kasuwa. Money is in whole kobo (integers)."""
+from decimal import ROUND_HALF_UP, Decimal

 RETURN_WINDOW_DAYS = 14
-RESTOCKING_FEE = 0.10  # charged on 'changed_mind' returns
+RESTOCKING_FEE = Decimal("0.10")  # charged on 'changed_mind' returns
 REASONS = {"failed_delivery", "damaged", "wrong_item", "changed_mind"}


+def round_kobo(amount):
+    """Round a Decimal amount of kobo to a whole kobo, halves up."""
+    return int(amount.quantize(Decimal("1"), rounding=ROUND_HALF_UP))
+
+
 def item_refund(quantity, unit_price_kobo, reason):
     """Refund for one order line, in kobo."""
-    naira = quantity * (unit_price_kobo / 100)
+    amount = Decimal(quantity * unit_price_kobo)
     if reason == "changed_mind":
-        naira = naira * (1 - RESTOCKING_FEE)
-    return int(naira * 100)
+        amount *= 1 - RESTOCKING_FEE
+    return round_kobo(amount)


 def within_window(delivered_on, today):
-    """True if a return is still allowed: within 14 days of delivery."""
-    return (today - delivered_on).days < RETURN_WINDOW_DAYS
+    """True if a return is allowed: on or before day 14 after delivery."""
+    return (today - delivered_on).days <= RETURN_WINDOW_DAYS


 def refund_amount(items, reason, delivery_fee_kobo):

Shell (bash)

%%bash
cat > test_issues.py <<'EOF'
from datetime import date

from refunds import item_refund, refund_amount, within_window


def test_damaged_refund_is_exact_to_the_kobo():
    # ISSUE-101: 249,999 + 150,000 = 399,999 kobo
    assert refund_amount([(1, 249_999)], "damaged", 150_000) == 399_999


def test_restocking_fee_rounds_half_up():
    # 249,999 x 0.9 = 224,999.1 -> 224,999;  333,333 x 0.9 = 299,999.7 -> 300,000
    assert item_refund(1, 249_999, "changed_mind") == 224_999
    assert item_refund(1, 333_333, "changed_mind") == 300_000


def test_return_on_day_14_is_allowed():
    # ISSUE-103: delivered 1 September, returned 15 September
    assert within_window(date(2026, 9, 1), date(2026, 9, 15))


def test_return_on_day_15_is_refused():
    assert not within_window(date(2026, 9, 1), date(2026, 9, 16))
EOF
python -m pytest
python -c "from refunds import refund_amount; print('K-1001 refund:', refund_amount([(1, 249_999)], 'damaged', 150_000), 'kobo')"

.......
7 passed in 0.01s
K-1001 refund: 399999 kobo