Python amount parsing: define a wire format before Decimal

Operation contract

The parser accepts a nonnegative canonical amount with exactly two ASCII fractional digits. It returns Decimal for 47.25 and rejects a comma-formatted value. The rule is explicit and independent of the machine's current locale setting.

Failure boundary

A canonical API field is not a localized display string. If users enter localized amounts, parse under a deliberate locale-specific interface before converting to the canonical contract. Do not call setlocale for one request in a multi-threaded server; it changes process-wide state. Currency scale and maximum amount remain separate business rules.

Working program

from decimal import Decimal

ASCII_DIGITS = set("0123456789")

def parse_payment_amount(raw_amount):
    whole, separator, cents = raw_amount.partition(".")
    if (separator != "." or not whole or len(cents) != 2
            or (len(whole) > 1 and whole.startswith("0"))
            or any(character not in ASCII_DIGITS for character in whole + cents)):
        raise ValueError("amount must use canonical decimal format")
    return Decimal(raw_amount)

print("accepted", parse_payment_amount("47.25"))
try:
    parse_payment_amount("47,25")
except ValueError:
    print("comma_rejected", True)

Output

accepted 47.25
comma_rejected True

Costs and limits

Validation scans O(n) characters before Decimal conversion and retains an O(n) numeric representation. A separate maximum-length check is needed for hostile input.

Common Mistakes

• Do not use the server locale as a payment wire format.
• Decimal does not decide which localized input shape is allowed.
• A syntax check does not enforce currency scale or maximum value.

Connected lessons

• Python Decimal money: parse decimal text and choose rounding explicitly
• Python Decimal Inexact trap: reject fractional minor units
• Python JSON validation: reject duplicate members and non-integer amounts

Test this contract.