Fix KeyError in Deezer track conversion when artist is missing (#6929)

Fixes #4339.

The album half of this issue is already fixed, `album_for_id` guards the
`contributors` key. This covers the remaining call site, `_get_track`,
which still assumes the key exists:

```python
artist, artist_id = self.get_artist(
    track_data.get("contributors", [track_data["artist"]])
)
```

Two problems. The default argument to `.get` is evaluated eagerly, so
`track_data["artist"]` runs on every call, even when `contributors` is
present. A track payload carrying `contributors` but no `artist` raises
KeyError, despite the code reading as though the fallback only applies
when `contributors` is missing. And the fallback itself depends on
`artist` being there, which is the same assumption sampsyo asked the
plugin to stop making.

I mirrored the shape already merged for the album path: use
`contributors` when present, fall back to `artist` when that's the one
we have, and leave the artist fields unset when neither key exists.
`str(artist_id)` now also only runs when there's an id, so a track with
no artist info gets `None` instead of the string "None". The album path
still calls `str(artist_id)` unconditionally and can store "None" the
same way, but I left it alone to keep this to the one call site. Happy
to follow up on it.

`_get_track` had no test coverage, so I added three tests: contributors
without artist, artist without contributors (the old fallback still
behaves the same), and neither key. The first and third fail on master
with KeyError at the `.get` line, and all three pass with the change.
ruff check and format are clean on both files.

beetsplug/deezer.py

@@ -187,16 +187,20 @@ class DeezerPlugin(SearchApiMetadataSourcePlugin[IDResponse]):
 
         :param track_data: Deezer Track object dict
         """
-        artist, artist_id = self.get_artist(
-            track_data.get("contributors", [track_data["artist"]])
-        )
+        contributors = track_data.get("contributors")
+        if contributors is None and (artist_data := track_data.get("artist")):
+            contributors = [artist_data]
+        if contributors is not None:
+            artist, artist_id = self.get_artist(contributors)
+        else:
+            artist, artist_id = None, None
         return TrackInfo(
             title=track_data["title"],
             track_id=track_data["id"],
             deezer_track_id=track_data["id"],
             isrc=track_data.get("isrc"),
             artist=artist,
-            artist_id=str(artist_id),
+            artist_id=str(artist_id) if artist_id is not None else None,
             length=track_data["duration"],
             index=track_data.get("track_position"),
             medium=track_data.get("disk_number"),

docs/changelog.rst

@@ -56,6 +56,11 @@ Bug fixes
   copyright/rights-statement text verbatim. It's now normalized to a concise
   label name, stripping copyright markers, years, and corporate, licensing, and
   territorial boilerplate. Affects both album and track metadata. :bug:`6796`
+- :doc:`plugins/deezer`: Track conversion no longer assumes the API sends both
+  ``contributors`` and ``artist``. The fallback to ``artist`` was evaluated even
+  when ``contributors`` was present, so a track payload without ``artist``
+  raised ``KeyError``. Albums were already guarded; this fixes the remaining
+  call site. :bug:`4339`
 
 ..
     For plugin developers

test/plugins/test_deezer.py

@@ -39,3 +39,36 @@ class TestSearchQuery:
         )
 
         assert query == 'album:"Album"'
+
+
+class TestGetTrack:
+    def track_data(self, **fields):
+        return {
+            "id": 1,
+            "title": "Title",
+            "duration": 100,
+            "link": "https://www.deezer.com/track/1",
+            **fields,
+        }
+
+    def test_uses_contributors_when_artist_is_missing(self, plugin):
+        track = plugin._get_track(
+            self.track_data(contributors=[{"id": 2, "name": "Artist"}])
+        )
+
+        assert track.artist == "Artist"
+        assert track.artist_id == "2"
+
+    def test_falls_back_to_artist_without_contributors(self, plugin):
+        track = plugin._get_track(
+            self.track_data(artist={"id": 2, "name": "Artist"})
+        )
+
+        assert track.artist == "Artist"
+        assert track.artist_id == "2"
+
+    def test_tolerates_missing_artist_and_contributors(self, plugin):
+        track = plugin._get_track(self.track_data())
+
+        assert track.artist is None
+        assert track.artist_id is None