mypy

    if len(charset) != len(set(charset)):

      raise ValueError("All values in charset must be unique.")

    self.charset = charset

    self.max_length = Optional[int]

    if max_length is not None:

      self.max_length = int(max_length)

    else:

      The shape is `(max_length, len(charset) + 1)`.

      The index of unknown character is `len(charset)`.

    """

    if self.max_length is not None:

    if isinstance(self.max_length, int):

      if (len(string) > self.max_length):  # Validation

        raise ValueError("The length of {} is longer than `max_length`.")

      string = self.pad_string(string)  # Padding