[downloader/ffmpeg] Allow passing custom arguments before -i

Closes #686

README.md

@@ -439,9 +439,12 @@ Then simply run `make`. You can also run `make yt-dlp` instead to compile only t
                                      (Alias: --external-downloader)
     --downloader-args NAME:ARGS      Give these arguments to the external
                                      downloader. Specify the downloader name and
-                                     the arguments separated by a colon ":". You
-                                     can use this option multiple times to give
-                                     different arguments to different downloaders
+                                     the arguments separated by a colon ":". For
+                                     ffmpeg, arguments can be passed to
+                                     different positions using the same syntax
+                                     as --postprocessor-args. You can use this
+                                     option multiple times to give different
+                                     arguments to different downloaders
                                      (Alias: --external-downloader-args)
 
 ## Filesystem Options:

yt_dlp/downloader/external.py

@@ -22,7 +22,7 @@ from ..utils import (
     cli_option,
     cli_valueless_option,
     cli_bool_option,
-    cli_configuration_args,
+    _configuration_args,
     encodeFilename,
     encodeArgument,
     handle_youtubedl_headers,
@@ -111,11 +111,10 @@ class ExternalFD(FileDownloader):
     def _valueless_option(self, command_option, param, expected_value=True):
         return cli_valueless_option(self.params, command_option, param, expected_value)
 
-    def _configuration_args(self, *args, **kwargs):
-        return cli_configuration_args(
-            self.params.get('external_downloader_args'),
-            [self.get_basename(), 'default'],
-            *args, **kwargs)
+    def _configuration_args(self, keys=None, *args, **kwargs):
+        return _configuration_args(
+            self.get_basename(), self.params.get('external_downloader_args'), self.get_basename(),
+            keys, *args, **kwargs)
 
     def _call_downloader(self, tmpfilename, info_dict):
         """ Either overwrite this or implement _make_cmd """
@@ -459,10 +458,10 @@ class FFmpegFD(ExternalFD):
             elif isinstance(conn, compat_str):
                 args += ['-rtmp_conn', conn]
 
-        for url in urls:
-            args += ['-i', url]
+        for i, url in enumerate(urls):
+            args += self._configuration_args((f'_i{i + 1}', '_i')) + ['-i', url]
 
-        args += self._configuration_args() + ['-c', 'copy']
+        args += ['-c', 'copy']
         if info_dict.get('requested_formats'):
             for (i, fmt) in enumerate(info_dict['requested_formats']):
                 if fmt.get('acodec') != 'none':
@@ -491,9 +490,10 @@ class FFmpegFD(ExternalFD):
         else:
             args += ['-f', EXT_TO_OUT_FORMATS.get(ext, ext)]
 
+        args += self._configuration_args((f'_o1', '_o', ''))
+
         args = [encodeArgument(opt) for opt in args]
         args.append(encodeFilename(ffpp._ffmpeg_filename_argument(tmpfilename), True))
-
         self._debug_cmd(args)
 
         proc = subprocess.Popen(args, stdin=subprocess.PIPE, env=env)

yt_dlp/options.py

@@ -742,12 +742,13 @@ def parseOpts(overrideArguments=None):
         metavar='NAME:ARGS', dest='external_downloader_args', default={}, type='str',
         action='callback', callback=_dict_from_options_callback,
         callback_kwargs={
-            'allowed_keys': '|'.join(list_external_downloaders()),
+            'allowed_keys': r'ffmpeg_[io]\d*|%s' % '|'.join(list_external_downloaders()),
             'default_key': 'default',
             'process': compat_shlex_split
         }, help=(
             'Give these arguments to the external downloader. '
             'Specify the downloader name and the arguments separated by a colon ":". '
+            'For ffmpeg, arguments can be passed to different positions using the same syntax as --postprocessor-args. '
             'You can use this option multiple times to give different arguments to different downloaders '
             '(Alias: --external-downloader-args)'))
 

yt_dlp/postprocessor/common.py

@@ -5,7 +5,7 @@ import os
 
 from ..compat import compat_str
 from ..utils import (
-    cli_configuration_args,
+    _configuration_args,
     encodeFilename,
     PostProcessingError,
 )
@@ -110,18 +110,9 @@ class PostProcessor(object):
         except Exception:
             self.report_warning(errnote)
 
-    def _configuration_args(self, exe, keys=None, default=[], use_compat=True):
-        pp_key = self.pp_key().lower()
-        exe = exe.lower()
-        root_key = exe if pp_key == exe else '%s+%s' % (pp_key, exe)
-        keys = ['%s%s' % (root_key, k) for k in (keys or [''])]
-        if root_key in keys:
-            keys += [root_key] + ([] if pp_key == exe else [(self.pp_key(), exe)]) + ['default']
-        else:
-            use_compat = False
-        return cli_configuration_args(
-            self.get_param('postprocessor_args'),
-            keys, default, use_compat)
+    def _configuration_args(self, exe, *args, **kwargs):
+        return _configuration_args(
+            self.pp_key(), self.get_param('postprocessor_args'), exe, *args, **kwargs)
 
 
 class AudioConversionError(PostProcessingError):

yt_dlp/utils.py

@@ -4987,6 +4987,18 @@ def cli_configuration_args(argdict, keys, default=[], use_compat=True):
             return [arg for args in arg_list for arg in args]
     return default
 
+def _configuration_args(main_key, argdict, exe, keys=None, default=[], use_compat=True):
+    main_key, exe = main_key.lower(), exe.lower()
+    root_key = exe if main_key == exe else f'{main_key}+{exe}'
+    keys = [f'{root_key}{k}' for k in (keys or [''])]
+    if root_key in keys:
+        if main_key != exe:
+            keys.append((main_key, exe))
+        keys.append('default')
+    else:
+        use_compat = False
+    return cli_configuration_args(argdict, keys, default, use_compat)
+
 
 class ISO639Utils(object):
     # See http://www.loc.gov/standards/iso639-2/ISO-639-2_utf-8.txt