code: def save_host_keys(self, filename):
          
  
          # update local host keys from file (in case other SSH clients
          # have written to the known_hosts file meanwhile.
          if self._host_keys_filename is not None:
              self.load_host_keys(self._host_keys_filename)
  
          with open(filename, 'w') as f:
              for hostname, keys in self._host_keys.items():
                  for keytype, key in keys.items():
                      f.write('%s %s %s\n' % (hostname, keytype, key.get_base64()))
docs: Save the host keys back to a file.  Only the host keys loaded with
          `load_host_keys` (plus any added directly) will be saved -- not any
          host keys loaded with `load_system_host_keys`.
  
          :param str filename: the filename to save to
  
          :raises IOError: if the file could not be written