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

def load_system_host_keys(self, filename=None):
        """
        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
        """
        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)