Check string is a valid ISBN number

def ISBNValidator(raw_isbn):
    """ Check string is a valid ISBN number"""
    isbn_to_check = raw_isbn.replace('-', '').replace(' ', '')

    if not isinstance(isbn_to_check, string_types):
        raise ValidationError(_(u'Invalid ISBN: Not a string'))

    if len(isbn_to_check) != 10 and len(isbn_to_check) != 13:
        raise ValidationError(_(u'Invalid ISBN: Wrong length'))
    
    if not isbn.is_valid(isbn_to_check):
        raise ValidationError(_(u'Invalid ISBN: Failed checksum'))

    if isbn_to_check != isbn_to_check.upper():
        raise ValidationError(_(u'Invalid ISBN: Only upper case allowed'))

    return True