Do not attempt to fetch lyrics with empty data

Modified `search_pairs` function in `lyrics.py` to:

* Firstly strip each of `artist`, `artist_sort` and `title` fields
* Only generate alternatives if both `artist` and `title` are not empty
* Ensure that `artist_sort` is not empty and not equal to artist (ignoring
  case) before appending it to the artists

Extended tests to cover the changes.

beetsplug/lyrics.py

@@ -163,7 +163,13 @@ def generate_alternatives(string, patterns):
                 alternatives.append(match.group(1))
         return alternatives
 
-    title, artist, artist_sort = item.title, item.artist, item.artist_sort
+    title, artist, artist_sort = (
+        item.title.strip(),
+        item.artist.strip(),
+        item.artist_sort.strip(),
+    )
+    if not title or not artist:
+        return ()
 
     patterns = [
         # Remove any featuring artists from the artists name
@@ -172,7 +178,7 @@ def generate_alternatives(string, patterns):
     artists = generate_alternatives(artist, patterns)
     # Use the artist_sort as fallback only if it differs from artist to avoid
     # repeated remote requests with the same search terms
-    if artist != artist_sort:
+    if artist_sort and artist.lower() != artist_sort.lower():
         artists.append(artist_sort)
 
     patterns = [

docs/changelog.rst

@@ -48,6 +48,9 @@ Bug fixes:
   configuration for each test case. This fixes the issue where some tests
   failed because they read developer's local lyrics configuration.
   :bug:`5133`
+* :doc:`plugins/lyrics`: Do not attempt to search for lyrics if either the
+  artist or title is missing and ignore ``artist_sort`` value if it is empty.
+  :bug:`2635`
 
 For packagers:
 

test/plugins/test_lyrics.py

@@ -33,21 +33,36 @@
 
 
 class TestLyricsUtils:
-    unexpected_empty_artist = pytest.mark.xfail(
-        reason="Empty artist '' should not be present"
+    @pytest.mark.parametrize(
+        "artist, title",
+        [
+            ("Artist", ""),
+            ("", "Title"),
+            (" ", ""),
+            ("", " "),
+            ("", ""),
+        ],
     )
+    def test_search_empty(self, artist, title):
+        actual_pairs = lyrics.search_pairs(Item(artist=artist, title=title))
+
+        assert not list(actual_pairs)
 
     @pytest.mark.parametrize(
         "artist, artist_sort, expected_extra_artists",
         [
-            _p("Alice ft. Bob", "", ["Alice"], marks=unexpected_empty_artist),
-            _p("Alice feat Bob", "", ["Alice"], marks=unexpected_empty_artist),
-            _p("Alice feat. Bob", "", ["Alice"], marks=unexpected_empty_artist),
-            _p("Alice feats Bob", "", [], marks=unexpected_empty_artist),
-            _p("Alice featuring Bob", "", ["Alice"], marks=unexpected_empty_artist),
-            _p("Alice & Bob", "", ["Alice"], marks=unexpected_empty_artist),
-            _p("Alice and Bob", "", ["Alice"], marks=unexpected_empty_artist),
-            _p("Alice", "", [], marks=unexpected_empty_artist),
+            ("Alice ft. Bob", "", ["Alice"]),
+            ("Alice feat Bob", "", ["Alice"]),
+            ("Alice feat. Bob", "", ["Alice"]),
+            ("Alice feats Bob", "", []),
+            ("Alice featuring Bob", "", ["Alice"]),
+            ("Alice & Bob", "", ["Alice"]),
+            ("Alice and Bob", "", ["Alice"]),
+            ("Alice", "", []),
+            ("Alice", "Alice", []),
+            ("Alice", "alice", []),
+            ("Alice", "alice ", []),
+            ("Alice", "Alice A", ["Alice A"]),
             ("CHVRCHΞS", "CHVRCHES", ["CHVRCHES"]),
             ("横山克", "Masaru Yokoyama", ["Masaru Yokoyama"]),
         ],