Iterator implementation.

def next(self):
        """Iterator implementation."""
        # pylint: disable=protected-access
        with self._pcontext._queuelock:
            # Check that the pool still deals with this loop.
            # pylint: disable=protected-access
            pool_loop_reached = max(self._pcontext._thread_loop_ids)
            if pool_loop_reached > self._loop_id or self._queue.empty():
                raise StopIteration()
            else:
                return self._queue.get()