Luhn Algorithm

A small, dependency-free Python implementation of the Luhn algorithm for validating and generating check digits.

The Luhn algorithm is commonly used for credit-card-like numbers, IMEI numbers, and other identifiers that need a simple typo-detection check. This project only checks the Luhn checksum; it does not verify issuers, account status, number lengths, or whether an identifier is real.

Features

• Validate complete numbers with is_valid
• Calculate the checksum for a complete number with checksum
• Generate a check digit with calculate_check_digit
• Append a valid check digit with append_check_digit
• Accept strings or integers
• Ignore common separators: spaces, hyphens, dots, slashes, backslashes, and commas

Requirements

• Python 3.8+
• No third-party packages

Quick Start

Validate a number from the command line:

python3 luhn.py 79927398713

Output:

valid

Invalid numbers print:

invalid

Run the test suite:

python3 -m unittest -v

Python Usage

from luhn import append_check_digit, calculate_check_digit, checksum, is_valid

print(is_valid("79927398713"))
# True

print(checksum("79927398713"))
# 0

print(calculate_check_digit("7992739871"))
# 3

print(append_check_digit("7992739871"))
# "79927398713"

Formatted values are normalized before validation:

is_valid("4539 1488 0343 6467")  # True
is_valid("6011-1111-1111-1117")  # True

API

Function	Returns	Description
is_valid(value)	bool	Returns True when a complete value has a valid Luhn checksum. Invalid input, such as letters or an empty string, returns False.
checksum(value)	int	Returns the Luhn checksum for a complete value. A result of 0 means the value is valid.
calculate_check_digit(payload)	int	Calculates the digit that should be appended to a payload that does not include a check digit yet.
append_check_digit(payload)	str	Returns the payload with its calculated check digit appended.

Input Rules

All public functions accept str and int values. The implementation removes these separators before processing:

space  -  .  /  \  ,

After normalization, the value must contain at least one digit and no other characters. checksum, calculate_check_digit, and append_check_digit raise ValueError for invalid input. is_valid catches that error and returns False, which makes it convenient for validation checks.

Examples

Create a valid identifier from a payload:

payload = "7992739871"
identifier = append_check_digit(payload)

assert identifier == "79927398713"
assert is_valid(identifier)

Inspect an invalid number:

value = "79927398714"

assert checksum(value) == 1
assert not is_valid(value)

Project Layout

.
├── luhn.py       # implementation and command-line entry point
├── test_luhn.py  # unittest coverage
└── README.md