Money and rounding

print(0.1 + 0.2)
print(round(2.675, 2))

0.30000000000000004
2.67

print(round(16.5), round(17.5), round(0.5))

16 18 0

from decimal import Decimal, ROUND_HALF_UP

VAT_RATE = Decimal("0.075")
MAX_DISCOUNT_PCT = 20

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 invoice_total(lines, discount_pct=0, vat_exempt=False):
    """Total in kobo. Lines are (quantity, unit price in kobo) pairs."""
    subtotal = sum(quantity * unit_price for quantity, unit_price in lines)
    after_discount = subtotal - round_kobo(Decimal(subtotal) * discount_pct / 100)
    vat = 0 if vat_exempt else round_kobo(after_discount * VAT_RATE)
    return after_discount + vat

print(invoice_total([(1, 220)]))     # 220 kobo; VAT is 16.5 kobo, which rounds up to 17

237

import pandas as pd

base = "https://academy.cloudtechanalytics.com/datasets/invoicing/"
lines = pd.read_csv(base + "invoice_lines.csv", dtype={"unit_price": str})
invoices = pd.read_csv(base + "invoices_raw.csv").drop_duplicates("invoice_id")
customers = pd.read_csv(base + "customers.csv")
invoices = invoices.merge(customers[["customer_id", "vat_exempt"]], on="customer_id", how="left")
bad_quantity = set(lines.loc[lines["quantity"] <= 0, "invoice_id"])
invoices = invoices[(invoices["discount_pct"] <= MAX_DISCOUNT_PCT) & invoices["vat_exempt"].notna() & ~invoices["invoice_id"].isin(bad_quantity)]

def to_kobo(naira):
    return round_kobo(Decimal(naira) * 100)

def old_total(rows, discount_pct, vat_exempt):
    total = sum(q * float(p) for q, p in rows)
    total = total - total * discount_pct / 100
    if not vat_exempt:
        total = total + total * 0.075
    return round(total, 2)

by_invoice = {k: list(zip(g["quantity"], g["unit_price"])) for k, g in lines.groupby("invoice_id")}
results = []
for inv in invoices.itertuples():
    rows = by_invoice[inv.invoice_id]
    new = invoice_total([(q, to_kobo(p)) for q, p in rows], inv.discount_pct, bool(inv.vat_exempt))
    old = round(old_total(rows, inv.discount_pct, bool(inv.vat_exempt)) * 100)
    results.append({"invoice_id": inv.invoice_id, "new_kobo": new, "old_kobo": old})
results = pd.DataFrame(results)
results["difference"] = results["old_kobo"] - results["new_kobo"]
print(len(results), "invoices checked")
results["difference"].value_counts().sort_index()

1171 invoices checked
difference
-1     108
 0    1007
 1      56
Name: count, dtype: int64