[youtube] Add alternative automatic captions extraction approach (Closes #8667)

youtube_dl/extractor/youtube.py

@@ -975,10 +975,8 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
             return {}
         try:
             args = player_config['args']
-            caption_url = args['ttsurl']
-            if not caption_url:
-                self._downloader.report_warning(err_msg)
-                return {}
+            caption_url = args.get('ttsurl')
+            if caption_url:
                 timestamp = args['timestamp']
                 # We get the available subtitles
                 list_params = compat_urllib_parse.urlencode({
@@ -1013,6 +1011,35 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
                         })
                     sub_lang_list[sub_lang] = sub_formats
                 return sub_lang_list
+
+            # Some videos don't provide ttsurl but rather caption_tracks and
+            # caption_translation_languages (e.g. 20LmZk1hakA)
+            caption_tracks = args['caption_tracks']
+            caption_translation_languages = args['caption_translation_languages']
+            caption_url = compat_parse_qs(caption_tracks.split(',')[0])['u'][0]
+            parsed_caption_url = compat_urlparse.urlparse(caption_url)
+            caption_qs = compat_parse_qs(parsed_caption_url.query)
+
+            sub_lang_list = {}
+            for lang in caption_translation_languages.split(','):
+                lang_qs = compat_parse_qs(compat_urllib_parse_unquote_plus(lang))
+                sub_lang = lang_qs.get('lc', [None])[0]
+                if not sub_lang:
+                    continue
+                sub_formats = []
+                for ext in self._SUBTITLE_FORMATS:
+                    caption_qs.update({
+                        'tlang': [sub_lang],
+                        'fmt': [ext],
+                    })
+                    sub_url = compat_urlparse.urlunparse(parsed_caption_url._replace(
+                        query=compat_urllib_parse.urlencode(caption_qs, True)))
+                    sub_formats.append({
+                        'url': sub_url,
+                        'ext': ext,
+                    })
+                sub_lang_list[sub_lang] = sub_formats
+            return sub_lang_list
         # An extractor error can be raise by the download process if there are
         # no automatic captions but there are subtitles
         except (KeyError, ExtractorError):