ISBN check digit solver, user feedback

Question

I am making an ISBN program to solve the check digit and I want to make it so that when the program finds the check digit for you it opens a new string saying "would you like to close the program or not", which I have already done so.

If they person says 'n' for no it returns back to the beginning and if the person says 'y' the program closes I got stuck and started to search the internet my code is below could anyone help adjust it thanks you.

HERES MY CODE:

ISBN=input("Please enter a 10 digit number for the ISBN check digit:  ")

while len(ISBN)!= 10:

    print("Please try again and make sure you entered 10 digits.")
    ISBN=int(input("Please enter the 10 digit number again: "))
    continue

else:
    D1 =int(ISBN[0])*11

    D2 =int(ISBN[1])*10
    D3 =int(ISBN[2])*9
    D4 =int(ISBN[3])*8
    D5 =int(ISBN[4])*7
    D6 =int(ISBN[5])*6
    D7 =int(ISBN[6])*5
    D8 =int(ISBN[7])*4
    D9 =int(ISBN[8])*3
    D10=int(ISBN[9])*2
    Sum=(D1+D2+D3+D4+D5+D6+D7+D8+D9+D10)
    Mod=Sum%11
    D11=11-Mod
    if D11==10:
        D11='X'
    ISBNNumber=str(ISBN)+str(D11)
    print("Your 11 digit ISBN Number is *" + ISBNNumber + "*")

def close():
    close=input ("would you like to close the program or try again 'y' for Yes and 'n' for No:")

    while len(close)==1:
        if input == "n":s
            return (ISBN)
        elif input == "y":
            exit()
close()#

Answer 1

The easiest way is to wrap everything in a while loop:

while True:
    # ... put all your code here
    close = input("Would you like to try again? Enter 'y' for Yes and 'n' for No: ")
    if close.lower() in ("n", "no"):
        print("Exiting")
        break

This will loop each time, unless the user enters 'n' (or similar). Note:

1. Clearer question, with an explicit yes or no answer; and
2. Use of lower and in to allow range of possible valid inputs.

More broadly, I think you have problems with your algorithm; the 10th character (check digit) for an ISBN-10 number is calculated based on the first nine: http://en.wikipedia.org/wiki/Check_digit#ISBN_10.

Answer 2

This adds a for loop in to your code

else:

Sum = 0
for i in range(len(isbn)):
    sum= int(isbn[i])
mod=sum%11
digit11=11-mod
if digit11==10:
   digit11='X'
iSBNNumber=str(isbn)+str(digit11)
print('Your 11 digit ISBN Number is ' + iSBNNumber)