Validate `--merge-output-format`

Closes #4489

README.md

@@ -859,10 +859,10 @@ You can also fork the project on github and run your fork's [build workflow](.gi
                                     downloadable
     -F, --list-formats              List available formats of each video.
                                     Simulate unless --no-simulate is used
-    --merge-output-format FORMAT    If a merge is required (e.g.
+    --merge-output-format FORMAT    Container to use when merging formats (e.g.
-                                    bestvideo+bestaudio), output to given
+                                    bestvideo+bestaudio). Ignored if no merge is
-                                    container format. One of mkv, mp4, ogg,
+                                    required. (currently supported: avi, flv,
-                                    webm, flv. Ignored if no merge is required
+                                    mkv, mov, mp4, webm)
 
 ## Subtitle Options:
     --write-subs                    Write subtitle file

yt_dlp/__init__.py

@@ -20,6 +20,7 @@ from .extractor.common import InfoExtractor
 from .options import parseOpts
 from .postprocessor import (
     FFmpegExtractAudioPP,
+    FFmpegMergerPP,
     FFmpegPostProcessor,
     FFmpegSubtitlesConvertorPP,
     FFmpegThumbnailsConvertorPP,
@@ -223,6 +224,7 @@ def validate_options(opts):
         validate_regex('format sorting', f, InfoExtractor.FormatSort.regex)
 
     # Postprocessor formats
+    validate_in('merge output format', opts.merge_output_format, FFmpegMergerPP.SUPPORTED_EXTS)
     validate_regex('audio format', opts.audioformat, FFmpegExtractAudioPP.FORMAT_RE)
     validate_in('subtitle format', opts.convertsubtitles, FFmpegSubtitlesConvertorPP.SUPPORTED_EXTS)
     validate_regex('thumbnail format', opts.convertthumbnails, FFmpegThumbnailsConvertorPP.FORMAT_RE)

yt_dlp/options.py

@@ -13,6 +13,7 @@ from .cookies import SUPPORTED_BROWSERS, SUPPORTED_KEYRINGS
 from .downloader.external import list_external_downloaders
 from .postprocessor import (
     FFmpegExtractAudioPP,
+    FFmpegMergerPP,
     FFmpegSubtitlesConvertorPP,
     FFmpegThumbnailsConvertorPP,
     FFmpegVideoRemuxerPP,
@@ -781,9 +782,8 @@ def create_parser():
         '--merge-output-format',
         action='store', dest='merge_output_format', metavar='FORMAT', default=None,
         help=(
-            'If a merge is required (e.g. bestvideo+bestaudio), '
+            'Container to use when merging formats (e.g. bestvideo+bestaudio). Ignored if no merge is required. '
-            'output to given container format. One of mkv, mp4, ogg, webm, flv. '
+            f'(currently supported: {", ".join(sorted(FFmpegMergerPP.SUPPORTED_EXTS))})'))
-            'Ignored if no merge is required'))
     video_format.add_option(
         '--allow-unplayable-formats',
         action='store_true', dest='allow_unplayable_formats', default=False,

yt_dlp/postprocessor/ffmpeg.py

@@ -801,6 +801,8 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
 
 
 class FFmpegMergerPP(FFmpegPostProcessor):
+    SUPPORTED_EXTS = MEDIA_EXTENSIONS.common_video
+
     @PostProcessor._restrict_to(images=False)
     def run(self, info):
         filename = info['filepath']