[ie/youtube] Fix `--mark-watched` support (#13222)

Closes #11532
Authored by: iednod55, brian6932

Co-authored-by: iednod55 <210167282+iednod55@users.noreply.github.com>

yt_dlp/extractor/youtube/_video.py

@@ -2402,6 +2402,11 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
         return sts
 
     def _mark_watched(self, video_id, player_responses):
+        # cpn generation algorithm is reverse engineered from base.js.
+        # In fact it works even with dummy cpn.
+        CPN_ALPHABET = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_'
+        cpn = ''.join(CPN_ALPHABET[random.randint(0, 256) & 63] for _ in range(16))
+
         for is_full, key in enumerate(('videostatsPlaybackUrl', 'videostatsWatchtimeUrl')):
             label = 'fully ' if is_full else ''
             url = get_first(player_responses, ('playbackTracking', key, 'baseUrl'),
@@ -2412,11 +2417,6 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
             parsed_url = urllib.parse.urlparse(url)
             qs = urllib.parse.parse_qs(parsed_url.query)
 
-            # cpn generation algorithm is reverse engineered from base.js.
-            # In fact it works even with dummy cpn.
-            CPN_ALPHABET = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_'
-            cpn = ''.join(CPN_ALPHABET[random.randint(0, 256) & 63] for _ in range(16))
-
             # # more consistent results setting it to right before the end
             video_length = [str(float((qs.get('len') or ['1.5'])[0]) - 1)]