convert: Refine and fix playlist feature

- Improve --help text
- Use unicode instead of bytes when adding media file paths to the
  playlist file.
- The "standard" (?) of m3u8 defines that unicode should ensure support
  of special characters in media file names. util.displayable_path() is
  used to do the conversion from bytes. We save everything in bytes in
  the config since it seemes to be the way this plugin or beets in
  general likes to save paths.
- Join dest and playlist in the config reader method already to have it
  ready in both methods that require the full path to the playlist file.

beetsplug/convert.py

@@ -149,7 +149,7 @@ class ConvertPlugin(BeetsPlugin):
             'copy_album_art': False,
             'album_art_maxwidth': 0,
             'delete_originals': False,
-            'playlist': '',
+            'playlist': None,
         })
         self.early_import_stages = [self.auto_convert, self.auto_convert_keep]
 
@@ -179,7 +179,14 @@ class ConvertPlugin(BeetsPlugin):
                               help='hardlink files that do not \
                               need transcoding. Overrides --link.')
         cmd.parser.add_option('-m', '--playlist', action='store',
-                              help='set the name of the playlist file to be created')
+                              help='''the name of an m3u8 playlist file to
+                              be created in the root of the destination folder.
+                              The m3u8 format ensures special characters
+                              support by using unicode to save media file
+                              paths. Relative paths are used to point to media
+                              files ensuring a working playlist when
+                              transferred to a different computer (eg. when
+                              opened from an external drive).''')
         cmd.parser.add_album_option()
         cmd.func = self.convert_func
         return [cmd]
@@ -260,7 +267,7 @@ class ConvertPlugin(BeetsPlugin):
                            util.displayable_path(source))
 
     def convert_item(self, dest_dir, keep_new, path_formats, fmt,
-                     pretend=False, link=False, hardlink=False, playlist=''):
+                     pretend=False, link=False, hardlink=False, playlist=None):
         """A pipeline thread that converts `Item` objects from a
         library.
         """
@@ -285,12 +292,18 @@ class ConvertPlugin(BeetsPlugin):
                     dest = replace_ext(dest, ext)
                 converted = dest
 
-            # Quick, dirty, playlist
-            # converted_filename = Path(str(converted)).name
-            converted_filename = os.path.basename(converted)
-            self._log.info("Appending to playlist file {0}".format(playlist))
-            with open(playlist, "ab") as playlist_file:
-                playlist_file.write(converted_filename + b"\n")
+            # When the playlist argument is passed, add the current filename to
+            # an m3u8 playlist file located in the destination folder.
+            if playlist:
+                self._log.debug(
+                    "Appending to playlist file {0}",
+                    util.displayable_path(playlist)
+                )
+                with open(playlist, "a") as playlist_file:
+                    # The classic m3u format doesn't support special characters
+                    # in media file paths, thus we use the m3u8 format which
+                    # requires media file paths to be unicode.
+                    playlist_file.write(util.displayable_path(dest) + "\n")
 
             # Ensure that only one thread tries to create directories at a
             # time. (The existence check is not atomic with the directory
@@ -472,11 +485,9 @@ class ConvertPlugin(BeetsPlugin):
                                     link, hardlink)
 
         if playlist:
-            print("################")
-            print(f"Playlist: {playlist}")
-            print("################")
+            self._log.info("Creating playlist file: {0}", playlist)
             if not pretend:
-                with open(playlist, "w") as playlist_file:
+                with open(os.path.join(dest, playlist), "w") as playlist_file:
                     playlist_file.write("#EXTM3U" + "\n")
 
         self._parallel_convert(dest, opts.keep_new, path_formats, fmt,
@@ -563,6 +574,8 @@ class ConvertPlugin(BeetsPlugin):
         fmt = opts.format or self.config['format'].as_str().lower()
 
         playlist = opts.playlist or self.config['playlist'].get()
+        if playlist is not None:
+            playlist = os.path.join(dest, util.bytestring_path(playlist))
 
         if opts.pretend is not None:
             pretend = opts.pretend