docs: def save_as(self, filename=None):
          """
          Dumps object contents into file on disk.
  
          Args:
            filename (optional): defaults to self.filename. If passed, self.filename
              will be updated to filename.
          """
          if filename is None:
              filename = self.filename
          if filename is None:
              filename = self.default_filename
          if filename is None:
              raise RuntimeError("Class '{}' has no default filename".format(self.__class__.__name__))
          self._do_save_as(filename)
          self.filename = filename