[YoutubeDL] format spec: fix handling of '/' with ','

When using 'bestvideo/best,bestaudio', 'bestvideo/best' must be set as the current_selector (instead of appending it to the selectors), otherwise when it gets the ',' it would append 'None' to the selectors.

test/test_YoutubeDL.py

@@ -253,6 +253,14 @@ class TestFormatSelection(unittest.TestCase):
         downloaded = ydl.downloaded_info_dicts[0]
         self.assertEqual(downloaded['format_id'], '38')
 
+        info_dict = _make_result(list(formats_order), extractor='youtube')
+        ydl = YDL({'format': 'bestvideo/best,bestaudio'})
+        yie = YoutubeIE(ydl)
+        yie._sort_formats(info_dict['formats'])
+        ydl.process_ie_result(info_dict)
+        downloaded_ids = [info['format_id'] for info in ydl.downloaded_info_dicts]
+        self.assertEqual(downloaded_ids, ['137', '141'])
+
         info_dict = _make_result(list(formats_order), extractor='youtube')
         ydl = YDL({'format': '(bestvideo[ext=mp4],bestvideo[ext=webm])+bestaudio'})
         yie = YoutubeIE(ydl)

youtube_dl/YoutubeDL.py

@@ -958,8 +958,7 @@ class YoutubeDL(object):
                     elif string == '/':
                         first_choice = current_selector
                         second_choice = _parse_format_selection(tokens, inside_choice=True)
-                        current_selector = None
-                        selectors.append(FormatSelector(PICKFIRST, (first_choice, second_choice), []))
+                        current_selector = FormatSelector(PICKFIRST, (first_choice, second_choice), [])
                     elif string == '[':
                         if not current_selector:
                             current_selector = FormatSelector(SINGLE, 'best', [])