Keep missing multi-value fields as None instead of empty list

Author: snejus

beets/autotag/hooks.py

@@ -97,15 +97,15 @@ def __init__(
         self.artist = artist
         self.artist_credit = artist_credit
         self.artist_id = artist_id
-        self.artists = artists or []
-        self.artists_credit = artists_credit or []
-        self.artists_ids = artists_ids or []
+        self.artists = artists
+        self.artists_credit = artists_credit
+        self.artists_ids = artists_ids
         self.artist_sort = artist_sort
-        self.artists_sort = artists_sort or []
+        self.artists_sort = artists_sort
         self.data_source = data_source
         self.data_url = data_url
         self.genre = None
-        self.genres = genres or []
+        self.genres = genres
         self.media = media
         self.update(kwargs)
 
@@ -160,7 +160,7 @@ def __init__(
         self.albumdisambig = albumdisambig
         self.albumstatus = albumstatus
         self.albumtype = albumtype
-        self.albumtypes = albumtypes or []
+        self.albumtypes = albumtypes
         self.asin = asin
         self.barcode = barcode
         self.catalognum = catalognum

docs/changelog.rst

@@ -44,6 +44,9 @@ Bug fixes
 - :doc:`plugins/zero`: When the ``omit_single_disc`` option is set,
   ``disctotal`` is zeroed alongside ``disc``.
 - :doc:`plugins/fetchart`: Prevent deletion of configured fallback cover art
+- In autotagging, initialise empty multi-valued fields with ``None`` instead of
+  empty list, which caused beets to overwrite existing metadata with empty list
+  values instead of leaving them unchanged. :bug:`6403`
 
 For plugin developers
 ~~~~~~~~~~~~~~~~~~~~~

test/plugins/test_musicbrainz.py

@@ -539,7 +539,7 @@ def test_no_genres(self):
         config["musicbrainz"]["genres"] = False
         release = self._make_release()
         d = self.mb.album_info(release)
-        assert d.genres == []
+        assert d.genres is None
 
     def test_ignored_media(self):
         config["match"]["ignored_media"] = ["IGNORED1", "IGNORED2"]

test/test_autotag.py

@@ -229,7 +229,6 @@ def setUp(self):
             },
         ]
 
-    @pytest.mark.xfail(reason="Album genres are not applied to tracks")
     def test_autotag_items(self):
         self._apply()