Don't assume the 'subtitlesformat' is set in the params dict (fixes #1750)

youtube_dl/YoutubeDL.py

@@ -640,7 +640,7 @@ class YoutubeDL(object):
             # subtitles download errors are already managed as troubles in relevant IE
             # that way it will silently go on when used with unsupporting IE
             subtitles = info_dict['subtitles']
-            sub_format = self.params.get('subtitlesformat')
+            sub_format = self.params.get('subtitlesformat', 'srt')
             for sub_lang in subtitles.keys():
                 sub = subtitles[sub_lang]
                 if sub is None:

youtube_dl/extractor/youtube.py

@@ -1098,7 +1098,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor, SubtitlesInfoExtractor):
             params = compat_urllib_parse.urlencode({
                 'lang': lang,
                 'v': video_id,
-                'fmt': self._downloader.params.get('subtitlesformat'),
+                'fmt': self._downloader.params.get('subtitlesformat', 'srt'),
                 'name': l[0].encode('utf-8'),
             })
             url = u'http://www.youtube.com/api/timedtext?' + params
@@ -1111,7 +1111,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor, SubtitlesInfoExtractor):
     def _get_available_automatic_caption(self, video_id, webpage):
         """We need the webpage for getting the captions url, pass it as an
            argument to speed up the process."""
-        sub_format = self._downloader.params.get('subtitlesformat')
+        sub_format = self._downloader.params.get('subtitlesformat', 'srt')
         self.to_screen(u'%s: Looking for automatic captions' % video_id)
         mobj = re.search(r';ytplayer.config = ({.*?});', webpage)
         err_msg = u'Couldn\'t find automatic captions for %s' % video_id