Allow images formats

Necessary for #343.

* They are identified by `vcodec=acodec='none'`
* These formats show as the worst in `-F`
* Any postprocessor that expects audio/video will be skipped
* `b*` and all related selectors will skip such formats
* This commit also does not add any selector for downloading such formats. They have to be explicitly requested by the `format_id`. Implementation of a selector is left for when #389 is resolved

yt_dlp/postprocessor/common.py

@@ -1,5 +1,6 @@
 from __future__ import unicode_literals
 
+import functools
 import os
 
 from ..compat import compat_str
@@ -67,6 +68,25 @@ class PostProcessor(object):
         """Sets the downloader for this PP."""
         self._downloader = downloader
 
+    @staticmethod
+    def _restrict_to(*, video=True, audio=True, images=True):
+        allowed = {'video': video, 'audio': audio, 'images': images}
+
+        def decorator(func):
+            @functools.wraps(func)
+            def wrapper(self, info):
+                format_type = (
+                    'video' if info['vcodec'] != 'none'
+                    else 'audio' if info['acodec'] != 'none'
+                    else 'images')
+                if allowed[format_type]:
+                    func(self, info)
+                else:
+                    self.to_screen('Skipping %s' % format_type)
+                    return [], info
+            return wrapper
+        return decorator
+
     def run(self, information):
         """Run the PostProcessor.
 

yt_dlp/postprocessor/embedthumbnail.py

@@ -16,6 +16,7 @@ try:
 except ImportError:
     has_mutagen = False
 
+from .common import PostProcessor
 from .ffmpeg import (
     FFmpegPostProcessor,
     FFmpegThumbnailsConvertorPP,
@@ -62,6 +63,7 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
     def _report_run(self, exe, filename):
         self.to_screen('%s: Adding thumbnail to "%s"' % (exe, filename))
 
+    @PostProcessor._restrict_to(images=False)
     def run(self, info):
         filename = info['filepath']
         temp_filename = prepend_extension(filename, 'temp')

yt_dlp/postprocessor/ffmpeg.py

@@ -310,6 +310,7 @@ class FFmpegExtractAudioPP(FFmpegPostProcessor):
         except FFmpegPostProcessorError as err:
             raise AudioConversionError(err.msg)
 
+    @PostProcessor._restrict_to(images=False)
     def run(self, information):
         path = information['filepath']
         orig_ext = information['ext']
@@ -419,6 +420,7 @@ class FFmpegVideoConvertorPP(FFmpegPostProcessor):
             return ['-c:v', 'libxvid', '-vtag', 'XVID']
         return []
 
+    @PostProcessor._restrict_to(images=False)
     def run(self, information):
         path, source_ext = information['filepath'], information['ext'].lower()
         target_ext = self._target_ext(source_ext)
@@ -456,6 +458,7 @@ class FFmpegEmbedSubtitlePP(FFmpegPostProcessor):
         super(FFmpegEmbedSubtitlePP, self).__init__(downloader)
         self._already_have_subtitle = already_have_subtitle
 
+    @PostProcessor._restrict_to(images=False)
     def run(self, information):
         if information['ext'] not in ('mp4', 'webm', 'mkv'):
             self.to_screen('Subtitles can only be embedded in mp4, webm or mkv files')
@@ -523,6 +526,7 @@ class FFmpegEmbedSubtitlePP(FFmpegPostProcessor):
 
 
 class FFmpegMetadataPP(FFmpegPostProcessor):
+    @PostProcessor._restrict_to(images=False)
     def run(self, info):
         metadata = {}
 
@@ -625,6 +629,7 @@ class FFmpegMetadataPP(FFmpegPostProcessor):
 
 
 class FFmpegMergerPP(FFmpegPostProcessor):
+    @PostProcessor._restrict_to(images=False)
     def run(self, info):
         filename = info['filepath']
         temp_filename = prepend_extension(filename, 'temp')
@@ -657,6 +662,7 @@ class FFmpegMergerPP(FFmpegPostProcessor):
 
 
 class FFmpegFixupStretchedPP(FFmpegPostProcessor):
+    @PostProcessor._restrict_to(images=False, audio=False)
     def run(self, info):
         stretched_ratio = info.get('stretched_ratio')
         if stretched_ratio is None or stretched_ratio == 1:
@@ -676,6 +682,7 @@ class FFmpegFixupStretchedPP(FFmpegPostProcessor):
 
 
 class FFmpegFixupM4aPP(FFmpegPostProcessor):
+    @PostProcessor._restrict_to(images=False, video=False)
     def run(self, info):
         if info.get('container') != 'm4a_dash':
             return [], info
@@ -694,6 +701,7 @@ class FFmpegFixupM4aPP(FFmpegPostProcessor):
 
 
 class FFmpegFixupM3u8PP(FFmpegPostProcessor):
+    @PostProcessor._restrict_to(images=False)
     def run(self, info):
         filename = info['filepath']
         if self.get_audio_codec(filename) == 'aac':
@@ -805,6 +813,7 @@ class FFmpegSplitChaptersPP(FFmpegPostProcessor):
             ['-ss', compat_str(chapter['start_time']),
              '-t', compat_str(chapter['end_time'] - chapter['start_time'])])
 
+    @PostProcessor._restrict_to(images=False)
     def run(self, info):
         chapters = info.get('chapters') or []
         if not chapters:

yt_dlp/postprocessor/sponskrub.py

@@ -41,6 +41,7 @@ class SponSkrubPP(PostProcessor):
                 return None
         return path
 
+    @PostProcessor._restrict_to(images=False)
     def run(self, information):
         if self.path is None:
             return [], information