code: def ISBNValidator(raw_isbn):
      
      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
docs: Check string is a valid ISBN number