runtimequestion 397 Software Architecture · medium
The invoice calculation as a pure core
An invoicing script mixed database reads, arithmetic and PDF output. Write its core, invoice_totals(lines, prices, tax_rate): pure arithmetic over plain data, no I/O, so it can be tested and reused for the PDF and the API.
Function
invoice_totals(lines, prices, tax_rate)linesa list of
{"sku": ..., "qty": ...}; prices maps SKU to unit priceReturns
{"subtotal": s, "tax": t, "total": s + t}, each rounded to 2 decimals (round the subtotal and tax first, then add)Raises
KeyError for a SKU with no priceExamples
callinvoice_totals([{'sku': 'SPO-11681', 'qty': 2}, {'sku': 'HOM-22847', 'qty': 1}], {'SPO-11681': 7.49, 'HOM-22847': 31}, 0.0825)returns{'subtotal': 45.98, 'tax': 3.79, 'total': 49.77}
callinvoice_totals([], {'SPO-11681': 7.49}, 0.1)returns{'subtotal': 0, 'tax': 0.0, 'total': 0.0}
callinvoice_totals([{'sku': 'A', 'qty': 3}], {'A': 10}, 0)returns{'subtotal': 30, 'tax': 0, 'total': 30}