Discard info_dict from memory if no longer needed

Closes #1399

yt_dlp/YoutubeDL.py

@@ -319,9 +319,14 @@ class YoutubeDL:
     default_search:    Prepend this string if an input url is not valid.
                        'auto' for elaborate guessing
     encoding:          Use this encoding instead of the system-specified.
-    extract_flat:      Do not resolve URLs, return the immediate result.
-                       Pass in 'in_playlist' to only show this behavior for
-                       playlist items.
+    extract_flat:      Whether to resolve and process url_results further
+                       * False:     Always process (default)
+                       * True:      Never process
+                       * 'in_playlist': Do not process inside playlist/multi_video
+                       * 'discard': Always process, but don't return the result
+                                    from inside playlist/multi_video
+                       * 'discard_in_playlist': Same as "discard", but only for
+                                    playlists (not multi_video)
     wait_for_video:    If given, wait for scheduled streams to become available.
                        The value should be a tuple containing the range
                        (min_secs, max_secs) to wait between retries
@@ -1725,6 +1730,12 @@ class YoutubeDL:
         self.to_screen(f'[{ie_result["extractor"]}] Playlist {title}: Downloading {n_entries} videos'
                        f'{format_field(ie_result, "playlist_count", " of %s")}')
 
+        keep_resolved_entries = self.params.get('extract_flat') != 'discard'
+        if self.params.get('extract_flat') == 'discard_in_playlist':
+            keep_resolved_entries = ie_result['_type'] != 'playlist'
+        if keep_resolved_entries:
+            self.write_debug('The information of all playlist entries will be held in memory')
+
         failures = 0
         max_failures = self.params.get('skip_playlist_after_errors') or float('inf')
         for i, (playlist_index, entry) in enumerate(entries):
@@ -1765,7 +1776,8 @@ class YoutubeDL:
                 self.report_error(
                     f'Skipping the remaining entries in playlist "{title}" since {failures} items failed extraction')
                 break
-            resolved_entries[i] = (playlist_index, entry_result)
+            if keep_resolved_entries:
+                resolved_entries[i] = (playlist_index, entry_result)
 
         # Update with processed data
         ie_result['requested_entries'], ie_result['entries'] = tuple(zip(*resolved_entries)) or ([], [])