code: def load_system_host_keys(self, filename=None):
          
          if filename is None:
              # try the user's .ssh key file, and mask exceptions
              filename = os.path.expanduser("~/.ssh/known_hosts")
              try:
                  self._system_host_keys.load(filename)
              except IOError:
                  pass
              return
          self._system_host_keys.load(filename)
docs: Load host keys from a system (read-only) file.  Host keys read with
          this method will not be saved back by `save_host_keys`.
  
          This method can be called multiple times.  Each new set of host keys
          will be merged with the existing set (new replacing old if there are
          conflicts).
  
          If ``filename`` is left as ``None``, an attempt will be made to read
          keys from the user's local "known hosts" file, as used by OpenSSH,
          and no exception will be raised if the file can't be read.  This is
          probably only useful on posix.
  
          :param str filename: the filename to read, or ``None``
  
          :raises: ``IOError`` --
              if a filename was provided and the file could not be read