[outtmpl] Add operator `&` for replacement text (#2012)

Authored by: PilzAdam

README.md

@@ -1076,6 +1076,8 @@ The field names themselves (the part inside the parenthesis) can also have some
 
 1. **Alternatives**: Alternate fields can be specified seperated with a `,`. Eg: `%(release_date>%Y,upload_date>%Y|Unknown)s`
 
+1. **Replacement**: A replacement value can specified using a `&` separator. If the field is *not* empty, this replacement value will be used instead of the actual field content. This is done after alternate fields are considered; thus the replacement is used if *any* of the alternative fields is *not* empty.
+
 1. **Default**: A literal default value can be specified for when the field is empty using a `|` seperator. This overrides `--output-na-template`. Eg: `%(uploader|Unknown)s`
 
 1. **More Conversions**: In addition to the normal format types `diouxXeEfFgGcrs`, `B`, `j`, `l`, `q` can be used for converting to **B**ytes, **j**son (flag `#` for pretty-printing), a comma seperated **l**ist (flag `#` for `\n` newline-seperated) and a string **q**uoted for the terminal (flag `#` to split a list into different arguments), respectively
@@ -1084,7 +1086,7 @@ The field names themselves (the part inside the parenthesis) can also have some
 
 To summarize, the general syntax for a field is:
 ```
-%(name[.keys][addition][>strf][,alternate][|default])[flags][width][.precision][length]type
+%(name[.keys][addition][>strf][,alternate][&replacement][|default])[flags][width][.precision][length]type
 ```
 
 Additionally, you can set different output templates for the various metadata files separately from the general output template by specifying the type of file followed by the template separated by a colon `:`. The different file types supported are `subtitle`, `thumbnail`, `description`, `annotation` (deprecated), `infojson`, `link`, `pl_thumbnail`, `pl_description`, `pl_infojson`, `chapter`. For example, `-o '%(title)s.%(ext)s' -o 'thumbnail:%(title)s\%(title)s.%(ext)s'`  will put the thumbnails in a folder with the same name as the video. If any of the templates (except default) is empty, that type of file will not be written. Eg: `--write-thumbnail -o "thumbnail:"` will write thumbnails only for playlists and not for video.
@@ -1252,6 +1254,9 @@ $ yt-dlp -o '%(playlist)s/%(playlist_index)s - %(title)s.%(ext)s' https://www.yo
 # Download YouTube playlist videos in separate directories according to their uploaded year
 $ yt-dlp -o '%(upload_date>%Y)s/%(title)s.%(ext)s' https://www.youtube.com/playlist?list=PLwiyx1dc3P2JR9N8gQaQN_BCvlSlap7re
 
+# Prefix playlist index with " - " separator, but only if it is available
+$ yt-dlp -o '%(playlist_index|)s%(playlist_index& - |)s%(title)s.%(ext)s' BaW_jenozKc https://www.youtube.com/user/TheLinuxFoundation/playlists
+
 # Download all playlists of YouTube channel/user keeping each playlist in separate directory:
 $ yt-dlp -o '%(uploader)s/%(playlist)s/%(playlist_index)s - %(title)s.%(ext)s' https://www.youtube.com/user/TheLinuxFoundation/playlists
 

test/test_YoutubeDL.py

@@ -836,6 +836,11 @@ class TestYoutubeDL(unittest.TestCase):
         test('%(title3)s', ('foo/bar\\test', 'foo_bar_test'))
         test('folder/%(title3)s', ('folder/foo/bar\\test', 'folder%sfoo_bar_test' % os.path.sep))
 
+        # Replacement
+        test('%(id&foo)s.bar', 'foo.bar')
+        test('%(title&foo)s.bar', 'NA.bar')
+        test('%(title&foo|baz)s.bar', 'baz.bar')
+
     def test_format_note(self):
         ydl = YoutubeDL()
         self.assertEqual(ydl._format_note({}), '')

yt_dlp/YoutubeDL.py

@@ -1055,7 +1055,8 @@ class YoutubeDL(object):
             (?P<fields>{field})
             (?P<maths>(?:{math_op}{math_field})*)
             (?:>(?P<strf_format>.+?))?
-            (?P<alternate>(?<!\\),[^|)]+)?
+            (?P<alternate>(?<!\\),[^|&)]+)?
+            (?:&(?P<replacement>.*?))?
             (?:\|(?P<default>.*?))?
             $'''.format(field=FIELD_RE, math_op=MATH_OPERATORS_RE, math_field=MATH_FIELD_RE))
 
@@ -1114,11 +1115,12 @@ class YoutubeDL(object):
             key = outer_mobj.group('key')
             mobj = re.match(INTERNAL_FORMAT_RE, key)
             initial_field = mobj.group('fields').split('.')[-1] if mobj else ''
-            value, default = None, na
+            value, replacement, default = None, None, na
             while mobj:
                 mobj = mobj.groupdict()
                 default = mobj['default'] if mobj['default'] is not None else default
                 value = get_value(mobj)
+                replacement = mobj['replacement']
                 if value is None and mobj['alternate']:
                     mobj = re.match(INTERNAL_FORMAT_RE, mobj['alternate'][1:])
                 else:
@@ -1128,7 +1130,7 @@ class YoutubeDL(object):
             if fmt == 's' and value is not None and key in field_size_compat_map.keys():
                 fmt = '0{:d}d'.format(field_size_compat_map[key])
 
-            value = default if value is None else value
+            value = default if value is None else value if replacement is None else replacement
 
             flags = outer_mobj.group('conversion') or ''
             str_fmt = f'{fmt[:-1]}s'