join.py

import ffmpeg

def join_artifacts():
    audio_in = ffmpeg.input('audio_out.mp3')
    video_in = ffmpeg.input('out.mp4')
    ffmpeg.concat(video_in.filter("subtitles", "audio_out.srt"), audio_in, v=1, a=1).output('av_out.mp4').run()

join_artifacts()