[YoutubeDL] format spec: add additional checks for invalid syntax

test/test_YoutubeDL.py

@@ -302,6 +302,16 @@ class TestFormatSelection(unittest.TestCase):
             downloaded = ydl.downloaded_info_dicts[0]
             self.assertEqual(downloaded['format_id'], f1['format_id'])
 
+    def test_invalid_format_specs(self):
+        def assert_syntax_error(format_spec):
+            ydl = YDL({'format': format_spec})
+            info_dict = _make_result([{'format_id': 'foo', 'url': TEST_URL}])
+            self.assertRaises(SyntaxError, ydl.process_ie_result, info_dict)
+
+        assert_syntax_error('bestvideo,,best')
+        assert_syntax_error('+bestaudio')
+        assert_syntax_error('bestvideo+')
+
     def test_format_filtering(self):
         formats = [
             {'format_id': 'A', 'filesize': 500, 'width': 1000},

youtube_dl/YoutubeDL.py

@@ -953,6 +953,8 @@ class YoutubeDL(object):
                         tokens.restore_last_token()
                         break
                     elif string == ',':
+                        if not current_selector:
+                            raise syntax_error('"," must follow a format selector', start)
                         selectors.append(current_selector)
                         current_selector = None
                     elif string == '/':
@@ -972,6 +974,8 @@ class YoutubeDL(object):
                     elif string == '+':
                         video_selector = current_selector
                         audio_selector = _parse_format_selection(tokens, inside_merge=True)
+                        if not video_selector or not audio_selector:
+                            raise syntax_error('"+" must be between two format selectors', start)
                         current_selector = FormatSelector(MERGE, (video_selector, audio_selector), [])
                     else:
                         raise syntax_error('Operator not recognized: "{0}"'.format(string), start)