luhn.py

# Python3 program to implement
# Luhn algorithm
# Returns true if given card
# number is valid
def checkLuhn(cardNo):
    
    nDigits = len(cardNo)
    nSum = 0
    isSecond = False
    
    for i in range(nDigits - 1, -1, -1):
        d = ord(cardNo[i]) - ord('0')
    
        if (isSecond == True):
            d = d * 2
 
        # We add two digits to handle
        # cases that make two digits after
        # doubling
        nSum += d // 10
        nSum += d % 10
 
        isSecond = not isSecond
    
    if (nSum % 10 == 0):
        return True
    else:
        return False
# Driver code  
if __name__=="__main__":
    
    #cardNo = "79927398713"
    cardNo = "5555555555554444"
    
    if (checkLuhn(cardNo)):
        print("This is a valid card")
    else:
        print("This is not a valid card")
# This code is contributed by rutvik_56