Check ISBN Validity (Just check if the last bit is not conflict with the algorithm)

Check_ISBN.py

# Copyright (C) 2011, Giovanni Di Milia
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
# 
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
# 
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

def is_valid_isbn(isbn):
    """ 
        Function that validates ISBNs
        Posted on
        http://www.dimilia.it/2011/09/02/python-code-to-validate-isbns/
        Based on 
        http://en.wikipedia.org/wiki/International_Standard_Book_Number
        @author: Giovanni Di Milia
    """

    #I remove the valid hyphens in the code if they exist
    isbn = isbn.replace('-', '')

    #Validation of ISBN 10
    if len(isbn) == 10:
        try:
            checksum = str(11 - ((int(isbn[0])*10 + int(isbn[1])*9 +\
                                 int(isbn[2])*8 + int(isbn[3])*7 + \
                                 int(isbn[4])*6 + int(isbn[5])*5 +\
                                int(isbn[6])*4 + int(isbn[7])*3 +\
                                 int(isbn[8])*2) % 11))
        except ValueError:
            #if I fails it means that there are letters, so the ISBN is not valid
            return False

        if checksum == '11':
            checksum = '0'
        elif checksum == '10':
            checksum = 'X'
        if checksum == isbn[9]:
            return True
        else:
            return False
    #Validation of ISBN 13
    elif len(isbn) == 13:
        try:
            checksum = str(10 - ((int(isbn[0]) + int(isbn[1])*3 + \
                                  int(isbn[2]) + int(isbn[3])*3 +\
                                   int(isbn[4]) + int(isbn[5])*3 +\
                                    int(isbn[6]) + int(isbn[7])*3  +\
                                    int(isbn[8]) + int(isbn[9])*3 + \
                                    int(isbn[10]) + int(isbn[11])*3) % 10))
        except ValueError:
            #if I fails it means that there are letters, so the ISBN is not valid
            return False

        if checksum == '10':
            checksum = '0'
        if checksum == isbn[12]:
            return True
        else:
            return False
    else:
        return False

# Here are some samples
print is_valid_isbn("0-9553010-0-9")
print is_valid_isbn("978-0-9553010-0-1")