lastgenre: Rework combine, stringify, count reduction

- Rename method from _combine_genres() to _combine_resolve_and_log() to
  make clear that it not only combines new and old genres but also
  resolves them (which in this plugin's wording means "do the magic" of
  canonicalizationm, whitelist checking and reducing to a configured
  genre count).

- Clarify in _resolve docstring that a possible outcome might be all
  genres being removed.

- Add an additional log message telling which existing genres are taken
  into account BEFORE "the magic happens".

- Rename _to_delimited_genre_string() to _format_and_stringify()

- Move count reduction logic to _resolve_genres()

- Fix and rename a test

beetsplug/lastgenre/__init__.py

@@ -197,9 +197,12 @@ class LastGenrePlugin(plugins.BeetsPlugin):
         - Optionally, if the 'prefer_specific' configuration is enabled, the
           list is sorted by the specificity (depth in the canonicalization tree)
           of the genres.
-        - Finally, the method filters the tag list, ensuring that only valid
-          genres (those that pass the _is_valid method) are returned. If a
-          whitelist is set, only genres in the whitelist are considered valid.
+        - The method then filters the tag list, ensuring that only valid
+          genres (those that pass the _is_valid method) are kept. If a
+          whitelist is set, only genres in the whitelist are considered valid
+          (which may even result in no genres at all being retained).
+        - Finally, the filtered list of genres, limited to
+          the configured count is returned.
         """
         if not tags:
             return []
@@ -238,7 +241,8 @@ class LastGenrePlugin(plugins.BeetsPlugin):
 
         # c14n only adds allowed genres but we may have had forbidden genres in
         # the original tags list
-        return [x for x in tags if self._is_valid(x)]
+        valid_tags = [x for x in tags if self._is_valid(x)]
+        return valid_tags[: self.config["count"].get(int)]
 
     def fetch_genre(self, lastfm_obj):
         """Return the genre for a pylast entity or None if no suitable genre
@@ -301,17 +305,14 @@ class LastGenrePlugin(plugins.BeetsPlugin):
 
     # Main processing: _get_genre() and helpers.
 
-    def _to_delimited_genre_string(self, tags: list[str]) -> str:
-        """Reduce tags list to configured count, format and return as delimited
-        string."""
-        separator = self.config["separator"].as_str()
-        max_count = self.config["count"].get(int)
-
-        genres = tags[:max_count]
+    def _format_and_stringify(self, tags: list[str]) -> str:
+        """Format to title_case if configured and return as delimited string."""
         if self.config["title_case"]:
-            genres = [g.title() for g in genres]
+            formatted = [tag.title() for tag in tags]
+        else:
+            formatted = tags
 
-        return separator.join(genres)
+        return self.config["separator"].as_str().join(formatted)
 
     def _get_existing_genres(self, obj: Union[Album, Item]) -> list[str]:
         """Return a list of genres for this Item or Album. Empty string genres
@@ -325,9 +326,12 @@ class LastGenrePlugin(plugins.BeetsPlugin):
         # Filter out empty strings
         return [g for g in item_genre if g]
 
-    def _combine_genres(self, old: list[str], new: list[str]) -> list[str]:
-        """Combine old and new genres."""
+    def _combine_resolve_and_log(
+        self, old: list[str], new: list[str]
+    ) -> list[str]:
+        """Combine old and new genres and process via _resolve_genres."""
         self._log.debug(f"fetched last.fm tags: {new}")
+        self._log.debug(f"existing genres taken into account: {old}")
         combined = old + new
         return self._resolve_genres(combined)
 
@@ -411,13 +415,15 @@ class LastGenrePlugin(plugins.BeetsPlugin):
 
         # Return with a combined or freshly fetched genre list.
         if new_genres:
-            resolved_genres = self._combine_genres(keep_genres, new_genres)
-            if any(resolved_genres):
+            resolved_genres = self._combine_resolve_and_log(
+                keep_genres, new_genres
+            )
+            if resolved_genres:
                 suffix = "whitelist" if self.whitelist else "any"
                 label += f", {suffix}"
                 if keep_genres:
                     label = f"keep + {label}"
-                return self._to_delimited_genre_string(resolved_genres), label
+                return self._format_and_stringify(resolved_genres), label
 
         # Nothing found, leave original.
         if obj.genre:

test/plugins/test_lastgenre.py

@@ -79,16 +79,12 @@ class LastGenrePluginTest(BeetsTestCase):
         self._setup_config(canonical="", whitelist={"rock"})
         assert self.plugin._resolve_genres(["delta blues"]) == []
 
-    def test_to_delimited_string(self):
-        """Keep the n first genres, format them and return a
-        separator-delimited string.
-        """
+    def test_format_and_stringify(self):
+        """Format genres list and return them as a separator-delimited string."""
         self._setup_config(count=2)
         assert (
-            self.plugin._to_delimited_genre_string(
-                ["jazz", "pop", "rock", "blues"]
-            )
-            == "Jazz, Pop"
+            self.plugin._format_and_stringify(["jazz", "pop", "rock", "blues"])
+            == "Jazz, Pop, Rock, Blues"
         )
 
     def test_count_c14n(self):