def add_done_callback(self, fn):
<EOL>def done_callback(future_passed_to_callback):<EOL><INDENT>return fn()<EOL><DEDENT>self._future.add_done_callback(done_callback)<EOL>
Adds a callback to be completed once future is done

        :parm fn: A callable that takes no arguments. Note that is different
            than concurrent.futures.Future.add_done_callback that requires
            a single argument for the future.