generate ffmpeg command to do xfade

xfadeffmpeg.py

#!/usr/bin/python3
# https://stackoverflow.com/a/66546687
# https://newtoypia.blogspot.com/2021/05/ffmpeg.html
# 一口氣串接爆多影片？ ffmpeg 拯救可憐的手指

import argparse, subprocess, re

def gen_filter(segments, effect='fade', duration=0.3):
    video_fades = ''
    # fail! audio_fades = '\\\n[0:a]aresample=async=1;'
    audio_fades = ''
    settb = ''
    last_fade_output = '0:v'
    last_audio_output = '0:a'

    video_length = 0
    file_lengths = [0]*len(segments)
    
    input_files = ''
    for i in range(len(segments)):
        settb += '\\\n[{:d}]settb=AVTB[{:d}:v];'.format(i,i)
        input_files += ' -i ' + segments[i]

    for i in range(len(segments)-1):
        # file_lengths[i] = float(ffmpeg.probe(segments[i])['format']['duration'])
        file_lengths[i] = float(
            subprocess.check_output(
                re.split('\s+', 'ffprobe -v error -show_entries format=duration -of default=noprint_wrappers=1:nokey=1') + [segments[i]]
            )
        )
        video_length += file_lengths[i]
        next_fade_output = 'v{:d}{:d}'.format(i, i+1)
        video_fades += '\\\n[{}][{:d}:v]xfade=transition={}:duration={:0.2f}:offset={:0.2f}'.format(
            last_fade_output, i+1, effect, duration, video_length - duration*(i+1)
        )
        video_fades += '[{}];'.format(next_fade_output) \
            if i < len(segments)-2 \
            else ',format=yuv420p;'
        last_fade_output = next_fade_output

        next_audio_output = 'a{:d}{:d}'.format(i, i+1)
        audio_fades += '\\\n[{}][{:d}:a]acrossfade=d={:0.2f}'.format(
            last_audio_output, i+1, duration
        )
        if (i) < len(segments)-2:
            audio_fades += '[{}];'.format(next_audio_output)
        last_audio_output = next_audio_output
        
    return 'ffmpeg {} -filter_complex "{}"'.format(
        input_files, settb + video_fades + audio_fades
    )

parser = argparse.ArgumentParser(
    description='generate parameters for xfade of ffmpeg',
    formatter_class=argparse.ArgumentDefaultsHelpFormatter)
parser.add_argument('-o', '--outfile', type=str, default='out.mp4',
    help='output file name')
parser.add_argument('-e', '--effect', type=str, default='fade',
    help='transition effect')
parser.add_argument('-d', '--duration', type=float, default=0.3,
    help='transition duration')
parser.add_argument('videofiles', nargs='*', help=u'v1.mp4 v2.mp4 ...')
args = parser.parse_args()

print(gen_filter(args.videofiles, effect=args.effect, duration=args.duration) + '\\\n ' + args.outfile)