def load_system_host_keys(self, filename=None):
if filename is None:<EOL><INDENT>filename = os.path.expanduser('<STR_LIT>')<EOL>try:<EOL><INDENT>self._system_host_keys.load(filename)<EOL><DEDENT>except IOError:<EOL><INDENT>pass<EOL><DEDENT>return<EOL><DEDENT>self._system_host_keys.load(filename)<EOL>
Load host keys from a system (read-only) file.  Host keys read with
this method will not be saved back by L{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 C{filename} is left as C{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 filename: the filename to read, or C{None}
@type filename: str

@raise IOError: if a filename was provided and the file could not be
    read