## Python, string, credit_card_validator.py

``````"""
Functions for testing the validity of credit card numbers.

https://en.wikipedia.org/wiki/Luhn_algorithm
"""

def validate_initial_digits(credit_card_number: str) -> bool:
"""
Function to validate initial digits of a given credit card number.
>>> valid = "4111111111111111 41111111111111 34 35 37 412345 523456 634567"
>>> all(validate_initial_digits(cc) for cc in valid.split())
True
>>> invalid = "14 25 76 32323 36111111111111"
>>> all(validate_initial_digits(cc) is False for cc in invalid.split())
True
"""
return credit_card_number.startswith(("34", "35", "37", "4", "5", "6"))

def luhn_validation(credit_card_number: str) -> bool:
"""
Function to luhn algorithm validation for a given credit card number.
>>> luhn_validation('4111111111111111')
True
>>> luhn_validation('36111111111111')
True
>>> luhn_validation('41111111111111')
False
"""
cc_number = credit_card_number
total = 0
half_len = len(cc_number) - 2
for i in range(half_len, -1, -2):
#  double the value of every second digit
digit = int(cc_number[i])
digit *= 2
# If doubling of a number results in a two digit number
# i.e greater than 9(e.g., 6 × 2 = 12),
# then add the digits of the product (e.g., 12: 1 + 2 = 3, 15: 1 + 5 = 6),
# to get a single digit number.
if digit > 9:
digit %= 10
digit += 1
cc_number = cc_number[:i] + str(digit) + cc_number[i + 1 :]
total += digit

# Sum up the remaining digits
for i in range(len(cc_number) - 1, -1, -2):
total += int(cc_number[i])

def validate_credit_card_number(credit_card_number: str) -> bool:
"""
Function to validate the given credit card number.
>>> validate_credit_card_number('4111111111111111')
4111111111111111 is a valid credit card number.
True
>>> validate_credit_card_number('helloworld\$')
helloworld\$ is an invalid credit card number because it has nonnumerical characters.
False
>>> validate_credit_card_number('32323')
32323 is an invalid credit card number because of its length.
False
>>> validate_credit_card_number('32323323233232332323')
32323323233232332323 is an invalid credit card number because of its length.
False
>>> validate_credit_card_number('36111111111111')
36111111111111 is an invalid credit card number because of its first two digits.
False
>>> validate_credit_card_number('41111111111111')
41111111111111 is an invalid credit card number because it fails the Luhn check.
False
"""
error_message = f"{credit_card_number} is an invalid credit card number because"
if not credit_card_number.isdigit():
print(f"{error_message} it has nonnumerical characters.")
return False

if not 13 <= len(credit_card_number) <= 16:
print(f"{error_message} of its length.")
return False

if not validate_initial_digits(credit_card_number):
print(f"{error_message} of its first two digits.")
return False

if not luhn_validation(credit_card_number):
print(f"{error_message} it fails the Luhn check.")
return False

print(f"{credit_card_number} is a valid credit card number.")
return True

if __name__ == "__main__":
import doctest

doctest.testmod()
validate_credit_card_number("4111111111111111")
validate_credit_card_number("32323")
``````