code: def _clean_isbn(isbn):
      
      if isinstance(isbn, basestring):
          isbn = list(isbn.lower())
  
          # filter digits and "x"
          isbn = filter(lambda x: x.isdigit() or x == "x", isbn)
  
      # convert ISBN to numbers
      return map(lambda x: 10 if x == "x" else int(x), isbn)
docs: Remove all non-digit and non "x" characters from given string.
  
      Args:
          isbn (str): isbn string, which will be cleaned.
  
      Returns:
          list: array of numbers (if "x" is found, it is converted to 10).