fix pull request lint comments

cleaned up implementation and comments

yt_dlp/YoutubeDL.py

@@ -4242,7 +4242,7 @@ class YoutubeDL:
             self.write_debug(f'Skipping writing {label} thumbnail')
             return ret
 
-        if not self._ensure_dir_exists(filename):
+        if thumbnails and not self._ensure_dir_exists(filename):
             return None
 
         for idx, t in list(enumerate(thumbnails))[::-1]:

yt_dlp/postprocessor/ffmpeg.py

@@ -1046,20 +1046,23 @@ class FFmpegSplitChaptersPP(FFmpegPostProcessor):
             destination,
             ['-ss', str(chapter['start_time']),
              '-t', str(chapter['end_time'] - chapter['start_time'])])
+
     # FFmpeg adds metadata about all chapters from parent file to all split m4a files.
     # This is incorrect since there must be only single chapter in each file after split.
     # Such behavior confuses players who think multiple chapters present
     def _set_out_opts(self, ext, chapter_title):
+        out_opts = [*self.stream_copy_opts()]
+        out_opts.extend(['-map_metadata', '0'])
+        # exclude chapters metadata but keep everything else
+        out_opts.extend(['-map_chapters', '-1'])
+        if not chapter_title:
+            return out_opts
+        # replace global title with chapter specific title in split files
         if ext == 'm4a':
-            return [
-                *self.stream_copy_opts(),
-                # For m4a ffmpeg copies all available parent track chapters to split tracks metadata
-                # And such behavior confuses players
-                # Wipe parent track metadata from split tracks and fill out only title
-                '-metadata', 'title={}'.format(chapter_title),
-                '-map_metadata','-1']
-        else:
-            return self.stream_copy_opts()   
+            out_opts.extend(['-metadata', 'title={}'.format(chapter_title)])
+        if ext == 'opus':
+            out_opts.extend(['-metadata:s', 'title={}'.format(chapter_title)])
+        return out_opts
 
     @PostProcessor._restrict_to(images=False)
     def run(self, info):
@@ -1075,7 +1078,7 @@ class FFmpegSplitChaptersPP(FFmpegPostProcessor):
         self.to_screen('Splitting video by chapters; %d chapters found' % len(chapters))
         for idx, chapter in enumerate(chapters):
             destination, opts = self._ffmpeg_args_for_chapter(idx + 1, chapter, info)
-            out_file_opts = self._set_out_opts(info['ext'], chapter['title'])
+            out_file_opts = self._set_out_opts(info['ext'], chapter.get('title', ''))
             self.real_run_ffmpeg([(in_file, opts)], [(destination, out_file_opts)])
         if in_file != info['filepath']:
             self._delete_downloaded_files(in_file, msg=None)