[outtmpl] Support `str.format` syntax inside replacements

Closes #6843

README.md

@@ -1246,7 +1246,7 @@ The field names themselves (the part inside the parenthesis) can also have some
 
 1. **Alternatives**: Alternate fields can be specified separated with a `,`. E.g. `%(release_date>%Y,upload_date>%Y|Unknown)s`
 
-1. **Replacement**: A replacement value can be 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. **Replacement**: A replacement value can be specified using a `&` separator according to the [`str.format` mini-language](https://docs.python.org/3/library/string.html#format-specification-mini-language). 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. E.g. `%(chapters&has chapters|no chapters)s`, `%(title&TITLE={:>20}|NO TITLE)s`
 
 1. **Default**: A literal default value can be specified for when the field is empty using a `|` separator. This overrides `--output-na-placeholder`. E.g. `%(uploader|Unknown)s`
 

test/test_YoutubeDL.py

@@ -822,7 +822,10 @@ class TestYoutubeDL(unittest.TestCase):
         test('%(title&foo|baz)s.bar', 'baz.bar')
         test('%(x,id&foo|baz)s.bar', 'foo.bar')
         test('%(x,title&foo|baz)s.bar', 'baz.bar')
-        test('%(title&\n|)s', '\n')
+        test('%(id&a\nb|)s', ('a\nb', 'a b'))
+        test('%(id&hi {:>10} {}|)s', 'hi       1234 1234')
+        test(R'%(id&{0} {}|)s', 'NA')
+        test(R'%(id&{0.1}|)s', 'NA')
 
         # Laziness
         def gen():

yt_dlp/YoutubeDL.py

@@ -21,7 +21,7 @@ import tokenize
 import traceback
 import unicodedata
 import urllib.request
-from string import ascii_letters
+from string import Formatter, ascii_letters
 
 from .cache import Cache
 from .compat import compat_os_name, compat_shlex_quote
@@ -1237,6 +1237,14 @@ class YoutubeDL:
                 return list(obj)
             return repr(obj)
 
+        class _ReplacementFormatter(Formatter):
+            def get_field(self, field_name, args, kwargs):
+                if field_name.isdigit():
+                    return args[0], -1
+                raise ValueError('Unsupported field')
+
+        replacement_formatter = _ReplacementFormatter()
+
         def create_key(outer_mobj):
             if not outer_mobj.group('has_key'):
                 return outer_mobj.group(0)
@@ -1258,7 +1266,13 @@ class YoutubeDL:
             if fmt == 's' and value is not None and key in field_size_compat_map.keys():
                 fmt = f'0{field_size_compat_map[key]:d}d'
 
-            value = default if value is None else value if replacement is None else replacement
+            if value is None:
+                value = default
+            elif replacement is not None:
+                try:
+                    value = replacement_formatter.format(replacement, value)
+                except ValueError:
+                    value = na
 
             flags = outer_mobj.group('conversion') or ''
             str_fmt = f'{fmt[:-1]}s'