I'm looking a way to merge the video and audio files downloaded with youtube-dl (All using Pythonista, obviously.)

Has anyone done this?
I found some objective-c code in StackOverflow for example:

https://stackoverflow.com/questions/14618362/how-to-merge-audio-in-our-video-file
https://stackoverflow.com/questions/13909400/merging-audio-with-video-objective-c

I picked the second but haven't gone too far.

ns_video_path = nsurl(video_path)
ns_audio_path = nsurl(audio_path)
ns_out_video_path = nsurl(out_video_path)

AVMutableComposition = ObjCClass('AVMutableComposition')
mixComposition = AVMutableComposition.composition()
nextClipStartTime = CMTime.in_dll(c,'kCMTimeZero')

videoAsset= ObjCClass('AVURLAsset').alloc().initWithURL_options_(ns_video_path,None)

# How to call a objective-c  function? is there a bridge for them?  
# I reviewed objc_util http://omz-software.com/pythonista/docs/ios/objc_util.html
#
# CMTimeRange video_timeRange = CMTimeRangeMake(kCMTimeZero,videoAsset.duration);
#

Here's one sample from StackOverflow

-(void)mergeAudioVideo
{

   NSString *videoOutputPath=[_documentsDirectory stringByAppendingPathComponent:@"dummy_video.mp4"];
   NSString *outputFilePath = [_documentsDirectory stringByAppendingPathComponent:@"final_video.mp4"];
   if ([[NSFileManager defaultManager]fileExistsAtPath:outputFilePath])
       [[NSFileManager defaultManager]removeItemAtPath:outputFilePath error:nil];


   NSURL    *outputFileUrl = [NSURL fileURLWithPath:outputFilePath];
   NSString *filePath = [_documentsDirectory stringByAppendingPathComponent:@"newFile.m4a"];

   AVMutableComposition* mixComposition = [AVMutableComposition composition];
   NSURL    *audio_inputFileUrl = [NSURL fileURLWithPath:filePath];
   NSURL    *video_inputFileUrl = [NSURL fileURLWithPath:videoOutputPath];

   CMTime nextClipStartTime = kCMTimeZero;

   AVURLAsset* videoAsset = [[AVURLAsset alloc]initWithURL:video_inputFileUrl options:nil];
   CMTimeRange video_timeRange = CMTimeRangeMake(kCMTimeZero,videoAsset.duration);

   AVMutableCompositionTrack *a_compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
   [a_compositionVideoTrack insertTimeRange:video_timeRange ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:nextClipStartTime error:nil];

   AVURLAsset* audioAsset = [[AVURLAsset alloc]initWithURL:audio_inputFileUrl options:nil];
   CMTimeRange audio_timeRange = CMTimeRangeMake(kCMTimeZero, audioAsset.duration);
   AVMutableCompositionTrack *b_compositionAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
   [b_compositionAudioTrack insertTimeRange:audio_timeRange ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:nextClipStartTime error:nil];

   AVAssetExportSession* _assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetMediumQuality];
   _assetExport.outputFileType = @"com.apple.quicktime-movie";
   _assetExport.outputURL = outputFileUrl;

   [_assetExport exportAsynchronouslyWithCompletionHandler:
    ^(void ) {
        if (_assetExport.status == AVAssetExportSessionStatusCompleted) {

         //Write Code Here to Continue
        }
        else {
           //Write Fail Code here
        }
    }
    ];

}