Allow alternate fields in outtmpl

Closes #899, #1004

README.md

@@ -958,12 +958,13 @@ The field names themselves (the part inside the parenthesis) can also have some
 1. **Object traversal**: The dictionaries and lists available in metadata can be traversed by using a `.` (dot) separator. You can also do python slicing using `:`. Eg: `%(tags.0)s`, `%(subtitles.en.-1.ext)s`, `%(id.3:7:-1)s`, `%(formats.:.format_id)s`. `%()s` refers to the entire infodict. Note that all the fields that become available using this method are not listed below. Use `-j` to see such fields
 1. **Addition**: Addition and subtraction of numeric fields can be done using `+` and `-` respectively. Eg: `%(playlist_index+10)03d`, `%(n_entries+1-playlist_index)d`
 1. **Date/time Formatting**: Date/time fields can be formatted according to [strftime formatting](https://docs.python.org/3/library/datetime.html#strftime-and-strptime-format-codes) by specifying it separated from the field name using a `>`. Eg: `%(duration>%H-%M-%S)s`, `%(upload_date>%Y-%m-%d)s`, `%(epoch-3600>%H-%M-%S)s`
-1. **Default**: A default value can be specified for when the field is empty using a `|` seperator. This overrides `--output-na-template`. Eg: `%(uploader|Unknown)s`
+1. **Alternatives**: Alternate fields can be specified seperated with a `,`. Eg: `%(release_date>%Y,upload_date>%Y|Unknown)s`
+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, a comma seperated **l**ist and a string **q**uoted for the terminal respectively
 
 To summarize, the general syntax for a field is:
 ```
-%(name[.keys][addition][>strf][|default])[flags][width][.precision][length]type
+%(name[.keys][addition][>strf][,alternate][|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`, `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.

test/test_YoutubeDL.py

@@ -790,6 +790,12 @@ class TestYoutubeDL(unittest.TestCase):
         test('%(formats.0.id.-1+id)f', '1235.000000')
         test('%(formats.0.id.-1+formats.1.id.-1)d', '3')
 
+        # Alternates
+        test('%(title,id)s', '1234')
+        test('%(width-100,height+20|def)d', '1100')
+        test('%(width-100,height+width|def)s', 'def')
+        test('%(timestamp-x>%H\\,%M\\,%S,timestamp>%H\\,%M\\,%S)s', '12,00,00')
+
         # Laziness
         def gen():
             yield from range(5)

yt_dlp/YoutubeDL.py

@@ -955,6 +955,7 @@ class YoutubeDL(object):
             (?P<fields>{field})
             (?P<maths>(?:{math_op}{math_field})*)
             (?:>(?P<strf_format>.+?))?
+            (?P<alternate>(?<!\\),[^|)]+)?
             (?:\|(?P<default>.*?))?
             $'''.format(field=FIELD_RE, math_op=MATH_OPERATORS_RE, math_field=MATH_FIELD_RE))
 
@@ -996,7 +997,7 @@ class YoutubeDL(object):
                     operator = None
             # Datetime formatting
             if mdict['strf_format']:
-                value = strftime_or_none(value, mdict['strf_format'])
+                value = strftime_or_none(value, mdict['strf_format'].replace('\\,', ','))
 
             return value
 
@@ -1012,12 +1013,16 @@ class YoutubeDL(object):
                 return f'%{outer_mobj.group(0)}'
             key = outer_mobj.group('key')
             mobj = re.match(INTERNAL_FORMAT_RE, key)
-            if mobj is None:
-                value, default, mobj = None, na, {'fields': ''}
-            else:
+            initial_field = mobj.group('fields').split('.')[-1] if mobj else ''
+            value, default = None, na
+            while mobj:
                 mobj = mobj.groupdict()
-                default = mobj['default'] if mobj['default'] is not None else na
+                default = mobj['default'] if mobj['default'] is not None else default
                 value = get_value(mobj)
+                if value is None and mobj['alternate']:
+                    mobj = re.match(INTERNAL_FORMAT_RE, mobj['alternate'][1:])
+                else:
+                    break
 
             fmt = outer_mobj.group('format')
             if fmt == 's' and value is not None and key in field_size_compat_map.keys():
@@ -1052,7 +1057,7 @@ class YoutubeDL(object):
                     # So we convert it to repr first
                     value, fmt = repr(value), str_fmt
                 if fmt[-1] in 'csr':
-                    value = sanitize(mobj['fields'].split('.')[-1], value)
+                    value = sanitize(initial_field, value)
 
             key = '%s\0%s' % (key.replace('%', '%\0'), outer_mobj.group('format'))
             TMPL_DICT[key] = value