Returns a progress message for printing out to the console.

def progress_string(self):
        """Returns a progress message for printing out to the console."""

        if not self.last_result:
            return self._status_string()

        def location_string(hostname, pid):
            if hostname == os.uname()[1]:
                return "pid={}".format(pid)
            else:
                return "{} pid={}".format(hostname, pid)

        pieces = [
            "{}".format(self._status_string()), "[{}]".format(
                self.resources.summary_string()), "[{}]".format(
                    location_string(
                        self.last_result.get(HOSTNAME),
                        self.last_result.get(PID))), "{} s".format(
                            int(self.last_result.get(TIME_TOTAL_S)))
        ]

        if self.last_result.get(TRAINING_ITERATION) is not None:
            pieces.append("{} iter".format(
                self.last_result[TRAINING_ITERATION]))

        if self.last_result.get(TIMESTEPS_TOTAL) is not None:
            pieces.append("{} ts".format(self.last_result[TIMESTEPS_TOTAL]))

        if self.last_result.get(EPISODE_REWARD_MEAN) is not None:
            pieces.append("{} rew".format(
                format(self.last_result[EPISODE_REWARD_MEAN], ".3g")))

        if self.last_result.get(MEAN_LOSS) is not None:
            pieces.append("{} loss".format(
                format(self.last_result[MEAN_LOSS], ".3g")))

        if self.last_result.get(MEAN_ACCURACY) is not None:
            pieces.append("{} acc".format(
                format(self.last_result[MEAN_ACCURACY], ".3g")))

        return ", ".join(pieces)