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104 lines
3.4 KiB
Python
104 lines
3.4 KiB
Python
"""
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Functions for testing the validity of credit card numbers.
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https://en.wikipedia.org/wiki/Luhn_algorithm
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"""
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def validate_initial_digits(credit_card_number: str) -> bool:
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"""
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Function to validate initial digits of a given credit card number.
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>>> valid = "4111111111111111 41111111111111 34 35 37 412345 523456 634567"
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>>> all(validate_initial_digits(cc) for cc in valid.split())
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True
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>>> invalid = "14 25 76 32323 36111111111111"
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>>> all(validate_initial_digits(cc) is False for cc in invalid.split())
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True
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"""
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return credit_card_number.startswith(("34", "35", "37", "4", "5", "6"))
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def luhn_validation(credit_card_number: str) -> bool:
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"""
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Function to luhn algorithm validation for a given credit card number.
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>>> luhn_validation('4111111111111111')
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True
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>>> luhn_validation('36111111111111')
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True
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>>> luhn_validation('41111111111111')
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False
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"""
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cc_number = credit_card_number
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total = 0
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half_len = len(cc_number) - 2
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for i in range(half_len, -1, -2):
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# double the value of every second digit
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digit = int(cc_number[i])
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digit *= 2
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# If doubling of a number results in a two digit number
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# i.e greater than 9(e.g., 6 × 2 = 12),
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# then add the digits of the product (e.g., 12: 1 + 2 = 3, 15: 1 + 5 = 6),
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# to get a single digit number.
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if digit > 9:
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digit %= 10
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digit += 1
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cc_number = cc_number[:i] + str(digit) + cc_number[i + 1 :]
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total += digit
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# Sum up the remaining digits
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for i in range(len(cc_number) - 1, -1, -2):
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total += int(cc_number[i])
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return total % 10 == 0
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def validate_credit_card_number(credit_card_number: str) -> bool:
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"""
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Function to validate the given credit card number.
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>>> validate_credit_card_number('4111111111111111')
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4111111111111111 is a valid credit card number.
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True
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>>> validate_credit_card_number('helloworld$')
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helloworld$ is an invalid credit card number because it has nonnumerical characters.
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False
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>>> validate_credit_card_number('32323')
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32323 is an invalid credit card number because of its length.
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False
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>>> validate_credit_card_number('32323323233232332323')
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32323323233232332323 is an invalid credit card number because of its length.
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False
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>>> validate_credit_card_number('36111111111111')
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36111111111111 is an invalid credit card number because of its first two digits.
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False
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>>> validate_credit_card_number('41111111111111')
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41111111111111 is an invalid credit card number because it fails the Luhn check.
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False
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"""
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error_message = f"{credit_card_number} is an invalid credit card number because"
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if not credit_card_number.isdigit():
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print(f"{error_message} it has nonnumerical characters.")
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return False
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if not 13 <= len(credit_card_number) <= 16:
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print(f"{error_message} of its length.")
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return False
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if not validate_initial_digits(credit_card_number):
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print(f"{error_message} of its first two digits.")
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return False
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if not luhn_validation(credit_card_number):
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print(f"{error_message} it fails the Luhn check.")
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return False
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print(f"{credit_card_number} is a valid credit card number.")
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return True
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if __name__ == "__main__":
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import doctest
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doctest.testmod()
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validate_credit_card_number("4111111111111111")
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validate_credit_card_number("32323")
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