Add support for multiple tags for multiple values

MPD supports multiple tags for some values, such as the artist, the
composer, and the performer. (See
<https://mpd.readthedocs.io/en/latest/protocol.html#tags>.) There
might be multiple artists for a track, in which case mopidy-mpd would
concatenate their names into one value, separated by a semicolon. For
some users and use cases, this can be suboptimal; when tracking the
tracks you listen to on a platform such as Listenbrainz, tracks of
multiple artists do not contribute to the total number of listens of
one of the artists.

This commit changes this behaviour to use multiple tags for values
when applicable instead of concatenating the values.

mopidy_mpd/translator.py

@@ -42,10 +42,11 @@ def track_to_mpd_format(track, position=None, stream_title=None):
     result = [
         ("file", track.uri),
         ("Time", track.length and (track.length // 1000) or 0),
-        ("Artist", concat_multi_values(track.artists, "name")),
         ("Album", track.album and track.album.name or ""),
     ]
 
+    result += multi_tag_list(track.artists, "name", "Artist")
+
     if stream_title is not None:
         result.append(("Title", stream_title))
         if track.name:
@@ -69,9 +70,8 @@ def track_to_mpd_format(track, position=None, stream_title=None):
         result.append(("MUSICBRAINZ_ALBUMID", track.album.musicbrainz_id))
 
     if track.album is not None and track.album.artists:
-        result.append(
-            ("AlbumArtist", concat_multi_values(track.album.artists, "name"))
-        )
+        result += multi_tag_list(track.album.artists, "name", "AlbumArtist")
+
         musicbrainz_ids = concat_multi_values(
             track.album.artists, "musicbrainz_id"
         )
@@ -84,14 +84,10 @@ def track_to_mpd_format(track, position=None, stream_title=None):
             result.append(("MUSICBRAINZ_ARTISTID", musicbrainz_ids))
 
     if track.composers:
-        result.append(
-            ("Composer", concat_multi_values(track.composers, "name"))
-        )
+        result += multi_tag_list(track.composers, "name", "Composer")
 
     if track.performers:
-        result.append(
-            ("Performer", concat_multi_values(track.performers, "name"))
-        )
+        result += multi_tag_list(track.performers, "name", "Performer")
 
     if track.genre:
         result.append(("Genre", track.genre))
@@ -151,6 +147,28 @@ def concat_multi_values(models, attribute):
     )
 
 
+def multi_tag_list(models, attribute, tag):
+    """
+    Format Mopidy model values for output to MPD client in a list with one tag
+    per value.
+
+    :param models: the models
+    :type models: array of :class:`mopidy.models.Artist`,
+        :class:`mopidy.models.Album` or :class:`mopidy.models.Track`
+    :param attribute: the attribute to use
+    :type attribute: string
+    :param tag: the name of the tag
+    :type tag: string
+    :rtype: list of tuples of string and attribute value
+    """
+
+    return [
+        (tag, getattr(m, attribute))
+        for m in models
+        if getattr(m, attribute, None) is not None
+    ]
+
+
 def tracks_to_mpd_format(tracks, start=0, end=None):
     """
     Format list of tracks for output to MPD client.

tests/test_translator.py

@@ -9,17 +9,23 @@
 class TrackMpdFormatTest(unittest.TestCase):
     track = Track(
         uri="à uri",
-        artists=[Artist(name="an artist")],
+        artists=[Artist(name="an artist"), Artist(name="yet another artist")],
         name="a nàme",
         album=Album(
             name="an album",
             num_tracks=13,
-            artists=[Artist(name="an other artist")],
+            artists=[
+                Artist(name="an other artist"),
+                Artist(name="still another artist"),
+            ],
             uri="urischeme:àlbum:12345",
         ),
         track_no=7,
-        composers=[Artist(name="a composer")],
-        performers=[Artist(name="a performer")],
+        composers=[Artist(name="a composer"), Artist(name="another composer")],
+        performers=[
+            Artist(name="a performer"),
+            Artist(name="another performer"),
+        ],
         genre="a genre",
         date="1977-01-01",
         disc_no=1,
@@ -69,11 +75,15 @@ def test_track_to_mpd_format_for_nonempty_track(self):
         assert ("file", "à uri") in result
         assert ("Time", 137) in result
         assert ("Artist", "an artist") in result
+        assert ("Artist", "yet another artist") in result
         assert ("Title", "a nàme") in result
         assert ("Album", "an album") in result
         assert ("AlbumArtist", "an other artist") in result
+        assert ("AlbumArtist", "still another artist") in result
         assert ("Composer", "a composer") in result
+        assert ("Composer", "another composer") in result
         assert ("Performer", "a performer") in result
+        assert ("Performer", "another performer") in result
         assert ("Genre", "a genre") in result
         assert ("Track", "7/13") in result
         assert ("Date", "1977-01-01") in result
@@ -82,7 +92,7 @@ def test_track_to_mpd_format_for_nonempty_track(self):
         assert ("Id", 122) in result
         assert ("X-AlbumUri", "urischeme:àlbum:12345") in result
         assert ("Comment", "a comment") not in result
-        assert len(result) == 15
+        assert len(result) == 19
 
     def test_track_to_mpd_format_with_last_modified(self):
         track = self.track.replace(last_modified=995303899000)