Add format type `B` for outtmpl to treat the value as bytes

This is useful to limit the filename to a certain number of bytes rather than characters
Closes #1003

README.md

@@ -952,14 +952,14 @@ The `-o` option is used to indicate a template for the output file names while `
 
 The simplest usage of `-o` is not to set any template arguments when downloading a single file, like in `yt-dlp -o funny_video.flv "https://some/video"` (hard-coding file extension like this is _not_ recommended and could break some post-processing).
 
-It may however also contain special sequences that will be replaced when downloading each video. The special sequences may be formatted according to [python string formatting operations](https://docs.python.org/2/library/stdtypes.html#string-formatting). For example, `%(NAME)s` or `%(NAME)05d`. To clarify, that is a percent symbol followed by a name in parentheses, followed by formatting operations.
+It may however also contain special sequences that will be replaced when downloading each video. The special sequences may be formatted according to [python string formatting operations](https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting). For example, `%(NAME)s` or `%(NAME)05d`. To clarify, that is a percent symbol followed by a name in parentheses, followed by formatting operations.
 
 The field names themselves (the part inside the parenthesis) can also have some special formatting:
 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. **More Conversions**: In addition to the normal format types `diouxXeEfFgGcrs`, `j`, `l`, `q` can be used for converting to **j**son, a comma seperated **l**ist and a string **q**uoted for the terminal respectively
+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:
 ```

test/test_YoutubeDL.py

@@ -649,6 +649,7 @@ class TestYoutubeDL(unittest.TestCase):
         'title2': '%PATH%',
         'title3': 'foo/bar\\test',
         'title4': 'foo "bar" test',
+        'title5': 'áéí',
         'timestamp': 1618488000,
         'duration': 100000,
         'playlist_index': 1,
@@ -767,6 +768,7 @@ class TestYoutubeDL(unittest.TestCase):
         test('%(ext)l', 'mp4')
         test('%(formats.:.id) 15l', '  id1, id2, id3')
         test('%(formats)j', (json.dumps(FORMATS), sanitize(json.dumps(FORMATS))))
+        test('%(title5).3B', 'á')
         if compat_os_name == 'nt':
             test('%(title4)q', ('"foo \\"bar\\" test"', "'foo _'bar_' test'"))
         else:

yt_dlp/YoutubeDL.py

@@ -907,7 +907,7 @@ class YoutubeDL(object):
     def validate_outtmpl(cls, outtmpl):
         ''' @return None or Exception object '''
         outtmpl = re.sub(
-            STR_FORMAT_RE_TMPL.format('[^)]*', '[ljq]'),
+            STR_FORMAT_RE_TMPL.format('[^)]*', '[ljqB]'),
             lambda mobj: f'{mobj.group(0)[:-1]}s',
             cls._outtmpl_expandpath(outtmpl))
         try:
@@ -939,7 +939,7 @@ class YoutubeDL(object):
         }
 
         TMPL_DICT = {}
-        EXTERNAL_FORMAT_RE = re.compile(STR_FORMAT_RE_TMPL.format('[^)]*', f'[{STR_FORMAT_TYPES}ljq]'))
+        EXTERNAL_FORMAT_RE = re.compile(STR_FORMAT_RE_TMPL.format('[^)]*', f'[{STR_FORMAT_TYPES}ljqB]'))
         MATH_FUNCTIONS = {
             '+': float.__add__,
             '-': float.__sub__,
@@ -1031,6 +1031,9 @@ class YoutubeDL(object):
                 value, fmt = json.dumps(value, default=_dumpjson_default), str_fmt
             elif fmt[-1] == 'q':
                 value, fmt = compat_shlex_quote(str(value)), str_fmt
+            elif fmt[-1] == 'B':
+                value = f'%{str_fmt}'.encode('utf-8') % str(value).encode('utf-8')
+                value, fmt = value.decode('utf-8', 'ignore'), 's'
             elif fmt[-1] == 'c':
                 value = str(value)
                 if value is None: