Support --output-format (and -O)

Allow specifying an output format when using an external downloader,
either through --output-format or -O, in preference to the extractor's
default. This is particularly helpful as -O mkv, to allow Matroska
output from ffmpeg in cases where .mp4 isn't a great answer, such as
while trying to play during the download.

youtube_dl/YoutubeDL.py

@@ -253,6 +253,7 @@ class YoutubeDL(object):
                        Progress hooks are guaranteed to be called at least once
                        (with status "finished") if the download is successful.
     merge_output_format: Extension to use when merging formats.
+    outputformat:      Extension and format to use when using an external downloader
     fixup:             Automatically correct known faults of the file.
                        One of:
                        - "never": do nothing
@@ -635,6 +636,8 @@ class YoutubeDL(object):
                     template_dict['resolution'] = '%sp' % template_dict['height']
                 elif template_dict.get('width'):
                     template_dict['resolution'] = '%dx?' % template_dict['width']
+            if self.params.get('outputformat'):
+                template_dict['ext'] = self.params.get('outputformat')
 
             sanitize = lambda k, v: sanitize_filename(
                 compat_str(v),
@@ -1864,6 +1867,8 @@ class YoutubeDL(object):
                         if filename_real_ext == info_dict['ext']
                         else filename)
                     requested_formats = info_dict['requested_formats']
+                    if self.params.get('outputformat'):
+                        info_dict['ext'] = self.params.get('outputformat')
                     if self.params.get('merge_output_format') is None and not compatible_formats(requested_formats):
                         info_dict['ext'] = 'mkv'
                         self.report_warning(

youtube_dl/__init__.py

@@ -417,6 +417,7 @@ def _real_main(argv=None):
         'ffmpeg_location': opts.ffmpeg_location,
         'hls_prefer_native': opts.hls_prefer_native,
         'hls_use_mpegts': opts.hls_use_mpegts,
+        'outputformat': opts.outputformat,
         'external_downloader_args': external_downloader_args,
         'postprocessor_args': postprocessor_args,
         'cn_verification_proxy': opts.cn_verification_proxy,

youtube_dl/downloader/common.py

@@ -49,6 +49,7 @@ class FileDownloader(object):
     external_downloader_args:  A list of additional command-line arguments for the
                         external downloader.
     hls_use_mpegts:     Use the mpegts container for HLS videos.
+    outputformat:       Output format for downloader to use
 
     Subclasses of this one must re-define the real_download method.
     """

youtube_dl/downloader/external.py

@@ -292,18 +292,26 @@ class FFmpegFD(ExternalFD):
         if self.params.get('test', False):
             args += ['-fs', compat_str(self._TEST_FILE_SIZE)]
 
-        if protocol in ('m3u8', 'm3u8_native'):
+        if self.params.get('outputformat'):
+            args += ['-f', EXT_TO_OUT_FORMATS.get(self.params.get('outputformat'),
+                                                  self.params.get('outputformat'))]
+        elif protocol in ('m3u8', 'm3u8_native'):
             if self.params.get('hls_use_mpegts', False) or tmpfilename == '-':
                 args += ['-f', 'mpegts']
             else:
                 args += ['-f', 'mp4']
-                if (ffpp.basename == 'ffmpeg' and is_outdated_version(ffpp._versions['ffmpeg'], '3.2', False)) and (not info_dict.get('acodec') or info_dict['acodec'].split('.')[0] in ('aac', 'mp4a')):
-                    args += ['-bsf:a', 'aac_adtstoasc']
         elif protocol == 'rtmp':
             args += ['-f', 'flv']
         else:
             args += ['-f', EXT_TO_OUT_FORMATS.get(info_dict['ext'], info_dict['ext'])]
 
+        if (protocol in ('m3u8', 'm3u8_native') and
+           (ffpp.basename == 'ffmpeg' and
+           is_outdated_version(ffpp._versions['ffmpeg'], '3.2', False)) and
+           (not info_dict.get('acodec') or
+           info_dict['acodec'].split('.')[0] in ('aac', 'mp4a'))):
+                    args += ['-bsf:a', 'aac_adtstoasc']
+
         args = [encodeArgument(opt) for opt in args]
         args.append(encodeFilename(ffpp._ffmpeg_filename_argument(tmpfilename), True))
 

youtube_dl/options.py

@@ -516,6 +516,10 @@ def parseOpts(overrideArguments=None):
         '--external-downloader-args',
         dest='external_downloader_args', metavar='ARGS',
         help='Give these arguments to the external downloader')
+    downloader.add_option(
+        '-O', '--output-format',
+        metavar='FORMAT', dest='outputformat', default='',
+        help='Ask the downloader to encode the specified video format')
 
     workarounds = optparse.OptionGroup(parser, 'Workarounds')
     workarounds.add_option(