docs: def sanitize_parse_mode(mode):
      """
      Converts the given parse mode into an object with
      ``parse`` and ``unparse`` callable properties.
      """
      if not mode:
          return None
  
      if callable(mode):
          class CustomMode:
              @staticmethod
              def unparse(text, entities):
                  raise NotImplementedError
  
          CustomMode.parse = mode
          return CustomMode
      elif (all(hasattr(mode, x) for x in ('parse', 'unparse'))
            and all(callable(x) for x in (mode.parse, mode.unparse))):
          return mode
      elif isinstance(mode, str):
          try:
              return {
                  'md': markdown,
                  'markdown': markdown,
                  'htm': html,
                  'html': html
              }[mode.lower()]
          except KeyError:
              raise ValueError('Unknown parse mode {}'.format(mode))
      else:
          raise TypeError('Invalid parse mode type {}'.format(mode))